I am looking to find a clear explanation of what the "th开发者_如何转开发is" keyword does, and how to use it correctly.

It seems to behave strangely, and I don't fully understand why.

How does this work and when should it be used?

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this is a keyword in JavaScript that is a property of an execution context. Its main use is in functions and constructors. The rules for this are quite simple (if you stick to best practices).

Technical description of this in the specification

The ECMAScript standard defines this via the abstract operation (abbreviated AO) ResolveThisBinding:

The [AO] ResolveThisBinding […] determines the binding of the keyword this using the LexicalEnvironment of the running execution context. [Steps]:

1. Let envRec be GetThisEnvironment().
2. Return ? envRec.GetThisBinding().

Global Environment Records, module Environment Records, and function Environment Records each have their own GetThisBinding method.

The GetThisEnvironment AO finds the current running execution context’s LexicalEnvironment and finds the closest ascendant Environment Record (by iteratively accessing their [[OuterEnv]] properties) which has a this binding (i.e. HasThisBinding returns true). This process ends in one of the three Environment Record types.

The value of this often depends on whether code is in strict mode.

The return value of GetThisBinding reflects the value of this of the current execution context, so whenever a new execution context is established, this resolves to a distinct value. This can also happen when the current execution context is modified. The following subsections list the five cases where this can happen.

You can put the code samples in the AST explorer to follow along with specification details.

1. Global execution context in scripts

This is script code evaluated at the top level, e.g. directly inside a <script>:

<script>
// Global context
console.log(this); // Logs global object.

setTimeout(function(){
  console.log("Not global context");
});
</script>

When in the initial global execution context of a script, evaluating this causes GetThisBinding to take the following steps:

The GetThisBinding concrete method of a global Environment Record envRec […] [does this]:

1. Return envRec.[[GlobalThisValue]].

The [[GlobalThisValue]] property of a global Environment Record is always set to the host-defined global object, which is reachable via globalThis (window on Web, global on Node.js; Docs on MDN). Follow the steps of InitializeHostDefinedRealm to learn how the [[GlobalThisValue]] property comes to be.

2. Global execution context in modules

Modules have been introduced in ECMAScript 2015.

This applies to modules, e.g. when directly inside a <script type="module">, as opposed to a simple <script>.

When in the initial global execution context of a module, evaluating this causes GetThisBinding to take the following steps:

The GetThisBinding concrete method of a module Environment Record […] [does this]:

1. Return undefined.

In modules, the value of this is always undefined in the global context. Modules are implicitly in strict mode.

3. Entering eval code

There are two kinds of eval calls: direct and indirect. This distinction exists since the ECMAScript 5th edition.

• A direct eval call usually looks like eval(…); or (eval)(…); (or ((eval))(…);, etc.).¹ It’s only direct if the call expression fits a narrow pattern.²
• An indirect eval call involves calling the function reference eval in any other way. It could be eval?.(…), (…, eval)(…), window.eval(…), eval.call(…,…), etc. Given const aliasEval1 = eval; window.aliasEval2 = eval;, it would also be aliasEval1(…), aliasEval2(…). Separately, given const originalEval = eval; window.eval = (x) => originalEval(x);, calling eval(…) would also be indirect.

See chuckj’s answer to “(1, eval)('this') vs eval('this') in JavaScript?” and Dmitry Soshnikov’s ECMA-262-5 in detail – Chapter 2: Strict Mode (archived) for when you might use an indirect eval() call.

PerformEval executes the eval code. It creates a new declarative Environment Record as its LexicalEnvironment, which is where GetThisEnvironment gets the this value from.

Then, if this appears in eval code, the GetThisBinding method of the Environment Record found by GetThisEnvironment is called and its value returned.

And the created declarative Environment Record depends on whether the eval call was direct or indirect:

• In a direct eval, it will be based on the current running execution context’s LexicalEnvironment.
• In an indirect eval, it will be based on the [[GlobalEnv]] property (a global Environment Record) of the Realm Record which executed the indirect eval.

Which means:

• In a direct eval, the this value doesn’t change; it’s taken from the lexical scope that called eval.
• In an indirect eval, the this value is the global object (globalThis).

What about new Function? — new Function is similar to eval, but it doesn’t call the code immediately; it creates a function. A this binding doesn’t apply anywhere here, except when the function is called, which works normally, as explained in the next subsection.

4. Entering function code

Entering function code occurs when calling a function.

There are four categories of syntax to invoke a function.

• The EvaluateCall AO is performed for these three:³
  • Normal function calls
  • Optional chaining calls
  • Tagged templates
• And EvaluateNew is performed for this one:³
  • Constructor invocations

The actual function call happens at the Call AO, which is called with a thisValue determined from context; this argument is passed along in a long chain of call-related calls. Call calls the [[Call]] internal slot of the function. This calls PrepareForOrdinaryCall where a new function Environment Record is created:

A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a this binding. If a function is not an ArrowFunction function and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.

In addition, there is the [[ThisValue]] field in a function Environment Record:

This is the this value used for this invocation of the function.

The NewFunctionEnvironment call also sets the function environment’s [[ThisBindingStatus]] property.

[[Call]] also calls OrdinaryCallBindThis, where the appropriate thisArgument is determined based on:

• the original reference,
• the kind of the function, and
• whether or not the code is in strict mode.

Once determined, a final call to the BindThisValue method of the newly created function Environment Record actually sets the [[ThisValue]] field to the thisArgument.

Finally, this very field is where a function Environment Record’s GetThisBinding AO gets the value for this from:

The GetThisBinding concrete method of a function Environment Record envRec […] [does this]:

[…]
3. Return envRec.[[ThisValue]].

Again, how exactly the this value is determined depends on many factors; this was just a general overview. With this technical background, let’s examine all the concrete examples.

Arrow functions

When an arrow function is evaluated, the [[ThisMode]] internal slot of the function object is set to “lexical” in OrdinaryFunctionCreate.

At OrdinaryCallBindThis, which takes a function F:

1. Let thisMode be F.[[ThisMode]].
2. If thisMode is lexical, return NormalCompletion(undefined). […]

which just means that the rest of the algorithm which binds this is skipped. An arrow function does not bind its own this value.

So, what is this inside an arrow function, then? Looking back at ResolveThisBinding and GetThisEnvironment, the HasThisBinding method explicitly returns false.

The HasThisBinding concrete method of a function Environment Record envRec […] [does this]:

1. If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.

So the outer environment is looked up instead, iteratively. The process will end in one of the three environments that have a this binding.

This just means that, in arrow function bodies, this comes from the lexical scope of the arrow function, or in other words (from Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?):

Arrow functions don’t have their own this […] binding. Instead, [this identifier is] resolved in the lexical scope like any other variable. That means that inside an arrow function, this [refers] to the [value of this] in the environment the arrow function is defined in (i.e. “outside” the arrow function).

Function properties

In normal functions (function, methods), this is determined by how the function is called.

This is where these “syntax variants” come in handy.

Consider this object containing a function:

const refObj = {
    func: function(){
      console.log(this);
    }
  };

Alternatively:

const refObj = {
    func(){
      console.log(this);
    }
  };

In any of the following function calls, the this value inside func will be refObj.¹

• refObj.func()
• refObj["func"]()
• refObj?.func()
• refObj.func?.()
• refObj.func``

If the called function is syntactically a property of a base object, then this base will be the “reference” of the call, which, in usual cases, will be the value of this. This is explained by the evaluation steps linked above; for example, in refObj.func() (or refObj["func"]()), the CallMemberExpression is the entire expression refObj.func(), which consists of the MemberExpression refObj.func and the Arguments ().

But also, refObj.func and refObj play three roles, each:

• they’re both expressions,
• they’re both references, and
• they’re both values.

refObj.func as a value is the callable function object; the corresponding reference is used to determine the this binding.

The optional chaining and tagged template examples work very similarly: basically, the reference is everything before the ?.(), before the ``, or before the ().

EvaluateCall uses IsPropertyReference of that reference to determine if it is a property of an object, syntactically. It’s trying to get the [[Base]] property of the reference (which is e.g. refObj, when applied to refObj.func; or foo.bar when applied to foo.bar.baz). If it is written as a property, then GetThisValue will get this [[Base]] property and use it as the this value.

Note: Getters / Setters work the same way as methods, regarding this. Simple properties don’t affect the execution context, e.g. here, this is in global scope:

const o = {
    a: 1,
    b: this.a, // Is `globalThis.a`.
    [this.a]: 2 // Refers to `globalThis.a`.
  };

Calls without base reference, strict mode, and with

A call without a base reference is usually a function that isn’t called as a property. For example:

func(); // As opposed to `refObj.func();`.

This also happens when passing or assigning methods, or using the comma operator. This is where the difference between Reference Record and Value is relevant.

Note function j: following the specification, you will notice that j can only return the function object (Value) itself, but not a Reference Record. Therefore the base reference refObj is lost.

const g = (f) => f(); // No base ref.
const h = refObj.func;
const j = () => refObj.func;

g(refObj.func);
h(); // No base ref.
j()(); // No base ref.
(0, refObj.func)(); // Another common pattern to remove the base ref.

EvaluateCall calls Call with a thisValue of undefined here. This makes a difference in OrdinaryCallBindThis (F: the function object; thisArgument: the thisValue passed to Call):

1. Let thisMode be F.[[ThisMode]].

[…]

5. If thisMode is strict, let thisValue be thisArgument.
6. Else,
   1. If thisArgument is undefined or null, then
      1. Let globalEnv be calleeRealm.[[GlobalEnv]].
      2. […]
      3. Let thisValue be globalEnv.[[GlobalThisValue]].
   2. Else,
      1. Let thisValue be ! ToObject(thisArgument).
      2. NOTE: ToObject produces wrapper objects […].

[…]

Note: step 5 sets the actual value of this to the supplied thisArgument in strict mode — undefined in this case. In “sloppy mode”, an undefined or null thisArgument results in this being the global this value.

If IsPropertyReference returns false, then EvaluateCall takes these steps:

1. Let refEnv be ref.[[Base]].
2. Assert: refEnv is an Environment Record.
3. Let thisValue be refEnv.WithBaseObject().

This is where an undefined thisValue may come from: refEnv.WithBaseObject() is always undefined, except in with statements. In this case, thisValue will be the binding object.

There’s also Symbol.unscopables (Docs on MDN) to control the with binding behavior.

To summarize, so far:

function f1(){
  console.log(this);
}

function f2(){
  console.log(this);
}

function f3(){
  console.log(this);
}

const o = {
    f1,
    f2,
    [Symbol.unscopables]: {
      f2: true
    }
  };

f1(); // Logs `globalThis`.

with(o){
  f1(); // Logs `o`.
  f2(); // `f2` is unscopable, so this logs `globalThis`.
  f3(); // `f3` is not on `o`, so this logs `globalThis`.
}

and:

"use strict";

function f(){
  console.log(this);
}

f(); // Logs `undefined`.

// `with` statements are not allowed in strict-mode code.

Note that when evaluating this, it doesn’t matter where a normal function is defined.

.call, .apply, .bind, thisArg, and primitives

Another consequence of step 5 of OrdinaryCallBindThis, in conjunction with step 6.2 (6.b in the spec), is that a primitive this value is coerced to an object only in “sloppy” mode.

To examine this, let’s introduce another source for the this value: the three methods that override the this binding:⁴

• Function.prototype.apply(thisArg, argArray)
• Function.prototype. {call, bind} (thisArg, ...args)

.bind creates a bound function, whose this binding is set to thisArg and cannot change again. .call and .apply call the function immediately, with the this binding set to thisArg.

.call and .apply map directly to Call, using the specified thisArg. .bind creates a bound function with BoundFunctionCreate. These have their own [[Call]] method which looks up the function object’s [[BoundThis]] internal slot.

Examples of setting a custom this value:

function f(){
  console.log(this);
}

const myObj = {},
  g = f.bind(myObj),
  h = (m) => m();

// All of these log `myObj`.
g();
f.bind(myObj)();
f.call(myObj);
h(g);

For objects, this is the same in strict and non-strict mode.

Now, try to supply a primitive value:

function f(){
  console.log(this);
}

const myString = "s",
  g = f.bind(myString);

g();              // Logs `String { "s" }`.
f.call(myString); // Logs `String { "s" }`.

In non-strict mode, primitives are coerced to their object-wrapped form. It’s the same kind of object you get when calling Object("s") or new String("s"). In strict mode, you can use primitives:

"use strict";

function f(){
  console.log(this);
}

const myString = "s",
  g = f.bind(myString);

g();              // Logs `"s"`.
f.call(myString); // Logs `"s"`.

Libraries make use of these methods, e.g. jQuery sets the this to the DOM element selected here:

$("button").click(function(){
  console.log(this); // Logs the clicked button.
});

Constructors, classes, and new

When calling a function as a constructor using the new operator, EvaluateNew calls Construct, which calls the [[Construct]] method. If the function is a base constructor (i.e. not a class extends…{…}), it sets thisArgument to a new object created from the constructor’s prototype. Properties set on this in the constructor will end up on the resulting instance object. this is implicitly returned, unless you explicitly return your own non-primitive value.

A class is a new way of creating constructor functions, introduced in ECMAScript 2015.

function Old(a){
  this.p = a;
}

const o = new Old(1);

console.log(o);  // Logs `Old { p: 1 }`.

class New{
  constructor(a){
    this.p = a;
  }
}

const n = new New(1);

console.log(n); // Logs `New { p: 1 }`.

Class definitions are implicitly in strict mode:

class A{
  m1(){
    return this;
  }
  m2(){
    const m1 = this.m1;
    
    console.log(m1());
  }
}

new A().m2(); // Logs `undefined`.

super

The exception to the behavior with new is class extends…{…}, as mentioned above. Derived classes do not immediately set their this value upon invocation; they only do so once the base class is reached through a series of super calls (happens implicitly without an own constructor). Using this before calling super is not allowed.

Calling super calls the super constructor with the this value of the lexical scope (the function Environment Record) of the call. GetThisValue has a special rule for super calls. It uses BindThisValue to set this to that Environment Record.

class DerivedNew extends New{
  constructor(a, a2){
    // Using `this` before `super` results in a ReferenceError.
    super(a);
    this.p2 = a2;
  }
}

const n2 = new DerivedNew(1, 2);

console.log(n2); // Logs `DerivedNew { p: 1, p2: 2 }`.

5. Evaluating class fields

Instance fields and static fields were introduced in ECMAScript 2022.

When a class is evaluated, ClassDefinitionEvaluation is performed, modifying the running execution context. For each ClassElement:

• if a field is static, then this refers to the class itself,
• if a field is not static, then this refers to the instance.

Private fields (e.g. #x) and methods are added to a PrivateEnvironment.

Static blocks are currently a TC39 stage 3 proposal. Static blocks work the same as static fields and methods: this inside them refers to the class itself.

Note that in methods and getters / setters, this works just like in normal function properties.

class Demo{
  a = this;
  b(){
    return this;
  }
  static c = this;
  static d(){
    return this;
  }
  // Getters, setters, private modifiers are also possible.
}

const demo = new Demo;

console.log(demo.a, demo.b()); // Both log `demo`.
console.log(Demo.c, Demo.d()); // Both log `Demo`.

---

¹: (o.f)() is equivalent to o.f(); (f)() is equivalent to f(). This is explained in this 2ality article (archived). Particularly see how a ParenthesizedExpression is evaluated.

²: It must be a MemberExpression, must not be a property, must have a [[ReferencedName]] of exactly "eval", and must be the %eval% intrinsic object.

³: Whenever the specification says “Let ref be the result of evaluating X.”, then X is some expression that you need to find the evaluation steps for. For example, evaluating a MemberExpression or CallExpression is the result of one of these algorithms. Some of them result in a Reference Record.

⁴: There are also several other native and host methods that allow providing a this value, notably Array.prototype.map, Array.prototype.forEach, etc. that accept a thisArg as their second argument. Anyone can make their own methods to alter this like (func, thisArg) => func.bind(thisArg), (func, thisArg) => func.call(thisArg), etc. As always, MDN offers great documentation.

---

Just for fun, test your understanding with some examples

For each code snippet, answer the question: “What is the value of this at the marked line? Why?”.

To reveal the answers, click the gray boxes.

1. if(true){
     console.log(this); // What is `this` here?
   }
   

   globalThis. The marked line is evaluated in the initial global execution context.

2. const obj = {};
   
   function myFun(){
     return { // What is `this` here?
       "is obj": this === obj,
       "is globalThis": this === globalThis
     };
   }
   
   obj.method = myFun;
   
   console.log(obj.method());
   
      

   obj. When calling a function as a property of an object, it is called with the this binding set to the base of the reference obj.method, i.e. obj.

3. const obj = {
       myMethod: function(){
         return { // What is `this` here?
           "is obj": this === obj,
           "is globalThis": this === globalThis
         };
       }
     },
     myFun = obj.myMethod;
   
   console.log(myFun());
   
      

   globalThis. Since the function value myFun / obj.myMethod is not called off of an object, as a property, the this binding will be globalThis. This is different from Python, in which accessing a method (obj.myMethod) creates a bound method object.

4. const obj = {
       myFun: () => ({ // What is `this` here?
         "is obj": this === obj,
         "is globalThis": this === globalThis
       })
     };
   
   console.log(obj.myFun());
   
      

   globalThis. Arrow functions don’t create their own this binding. The lexical scope is the same as the initial global scope, so this is globalThis.

5. function myFun(){
     console.log(this); // What is `this` here?
   }
   
   const obj = {
       myMethod: function(){
         eval("myFun()");
       }
     };
   
   obj.myMethod();
   

   globalThis. When evaluating the direct eval call, this is obj. However, in the eval code, myFun is not called off of an object, so the this binding is set to the global object.

6. function myFun() {
     // What is `this` here?
     return {
       "is obj": this === obj,
       "is globalThis": this === globalThis
     };
   }
   
   const obj = {};
   
   console.log(myFun.call(obj));
   
      

   obj. The line myFun.call(obj); is invoking the special built-in function Function.prototype.call, which accepts thisArg as the first argument.

7. class MyCls{
     arrow = () => ({ // What is `this` here?
       "is MyCls": this === MyCls,
       "is globalThis": this === globalThis,
       "is instance": this instanceof MyCls
     });
   }
   
   console.log(new MyCls().arrow());
   
      

   It’s the instance of MyCls. Arrow functions don’t change the this binding, so it comes from lexical scope. Therefore, this is exactly the same as with the class fields mentioned above, like a = this;. Try changing it to static arrow. Do you get the result you expect?

---

The this keyword behaves differently in JavaScript compared to other languages. In Object Oriented languages, the this keyword refers to the current instance of the class. In JavaScript the value of this is determined by the invocation context of function (context.function()) and where it is called.

1. When used in global context

When you use this in global context, it is bound to global object (window in browser)

document.write(this);  //[object Window]

When you use this inside a function defined in the global context, this is still bound to global object since the function is actually made a method of global context.

function f1()
{
   return this;
}
document.write(f1());  //[object Window]

Above f1 is made a method of global object. Thus we can also call it on window object as follows:

function f()
{
    return this;
}

document.write(window.f()); //[object Window]

2. When used inside object method

When you use this keyword inside an object method, this is bound to the "immediate" enclosing object.

var obj = {
    name: "obj",
    f: function () {
        return this + ":" + this.name;
    }
};
document.write(obj.f());  //[object Object]:obj

Above I have put the word immediate in double quotes. It is to make the point that if you nest the object inside another object, then this is bound to the immediate parent.

var obj = {
    name: "obj1",
    nestedobj: {
        name:"nestedobj",
        f: function () {
            return this + ":" + this.name;
        }
    }            
}

document.write(obj.nestedobj.f()); //[object Object]:nestedobj

Even if you add function explicitly to the object as a method, it still follows above rules, that is this still points to the immediate parent object.

var obj1 = {
    name: "obj1",
}

function returnName() {
    return this + ":" + this.name;
}

obj1.f = returnName; //add method to object
document.write(obj1.f()); //[object Object]:obj1

3. When invoking context-less function

When you use this inside function that is invoked without any context (i.e. not on any object), it is bound to the global object (window in browser)(even if the function is defined inside the object) .

var context = "global";

var obj = {  
    context: "object",
    method: function () {                  
        function f() {
            var context = "function";
            return this + ":" +this.context; 
        };
        return f(); //invoked without context
    }
};

document.write(obj.method()); //[object Window]:global 

Trying it all with functions

We can try above points with functions too. However there are some differences.

• Above we added members to objects using object literal notation. We can add members to functions by using this. to specify them.
• Object literal notation creates an instance of object which we can use immediately. With function we may need to first create its instance using new operator.
• Also in an object literal approach, we can explicitly add members to already defined object using dot operator. This gets added to the specific instance only. However I have added variable to the function prototype so that it gets reflected in all instances of the function.

Below I tried out all the things that we did with Object and this above, but by first creating function instead of directly writing an object.

/********************************************************************* 
  1. When you add variable to the function using this keyword, it 
     gets added to the function prototype, thus allowing all function 
     instances to have their own copy of the variables added.
*********************************************************************/
function functionDef()
{
    this.name = "ObjDefinition";
    this.getName = function(){                
        return this+":"+this.name;
    }
}        

obj1 = new functionDef();
document.write(obj1.getName() + "<br />"); //[object Object]:ObjDefinition   

/********************************************************************* 
   2. Members explicitly added to the function protorype also behave 
      as above: all function instances have their own copy of the 
      variable added.
*********************************************************************/
functionDef.prototype.version = 1;
functionDef.prototype.getVersion = function(){
    return "v"+this.version; //see how this.version refers to the
                             //version variable added through 
                             //prototype
}
document.write(obj1.getVersion() + "<br />"); //v1

/********************************************************************* 
   3. Illustrating that the function variables added by both above 
      ways have their own copies across function instances
*********************************************************************/
functionDef.prototype.incrementVersion = function(){
    this.version = this.version + 1;
}
var obj2 = new functionDef();
document.write(obj2.getVersion() + "<br />"); //v1

obj2.incrementVersion();      //incrementing version in obj2
                              //does not affect obj1 version

document.write(obj2.getVersion() + "<br />"); //v2
document.write(obj1.getVersion() + "<br />"); //v1

/********************************************************************* 
   4. `this` keyword refers to the immediate parent object. If you 
       nest the object through function prototype, then `this` inside 
       object refers to the nested object not the function instance
*********************************************************************/
functionDef.prototype.nestedObj = { name: 'nestedObj', 
                                    getName1 : function(){
                                        return this+":"+this.name;
                                    }                            
                                  };

document.write(obj2.nestedObj.getName1() + "<br />"); //[object Object]:nestedObj

/********************************************************************* 
   5. If the method is on an object's prototype chain, `this` refers 
      to the object the method was called on, as if the method was on 
      the object.
*********************************************************************/
var ProtoObj = { fun: function () { return this.a } };
var obj3 = Object.create(ProtoObj); //creating an object setting ProtoObj
                                    //as its prototype
obj3.a = 999;                       //adding instance member to obj3
document.write(obj3.fun()+"<br />");//999
                                    //calling obj3.fun() makes 
                                    //ProtoObj.fun() to access obj3.a as 
                                    //if fun() is defined on obj3

4. When used inside constructor function.

When the function is used as a constructor (that is when it is called with new keyword), this inside function body points to the new object being constructed.

var myname = "global context";
function SimpleFun()
{
    this.myname = "simple function";
}

var obj1 = new SimpleFun(); //adds myname to obj1
//1. `new` causes `this` inside the SimpleFun() to point to the
//   object being constructed thus adding any member
//   created inside SimipleFun() using this.membername to the
//   object being constructed
//2. And by default `new` makes function to return newly 
//   constructed object if no explicit return value is specified

document.write(obj1.myname); //simple function

5. When used inside function defined on prototype chain

If the method is on an object's prototype chain, this inside such method refers to the object the method was called on, as if the method is defined on the object.

var ProtoObj = {
    fun: function () {
        return this.a;
    }
};
//Object.create() creates object with ProtoObj as its
//prototype and assigns it to obj3, thus making fun() 
//to be the method on its prototype chain

var obj3 = Object.create(ProtoObj);
obj3.a = 999;
document.write(obj3.fun()); //999

//Notice that fun() is defined on obj3's prototype but 
//`this.a` inside fun() retrieves obj3.a   

6. Inside call(), apply() and bind() functions

• All these methods are defined on Function.prototype.
• These methods allows to write a function once and invoke it in different context. In other words, they allows to specify the value of this which will be used while the function is being executed. They also take any parameters to be passed to the original function when it is invoked.
• fun.apply(obj1 [, argsArray]) Sets obj1 as the value of this inside fun() and calls fun() passing elements of argsArray as its arguments.
• fun.call(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]]) - Sets obj1 as the value of this inside fun() and calls fun() passing arg1, arg2, arg3, ... as its arguments.
• fun.bind(obj1 [, arg1 [, arg2 [,arg3 [, ...]]]]) - Returns the reference to the function fun with this inside fun bound to obj1 and parameters of fun bound to the parameters specified arg1, arg2, arg3,....
• By now the difference between apply, call and bind must have become apparent. apply allows to specify the arguments to function as array-like object i.e. an object with a numeric length property and corresponding non-negative integer properties. Whereas call allows to specify the arguments to the function directly. Both apply and call immediately invokes the function in the specified context and with the specified arguments. On the other hand, bind simply returns the function bound to the specified this value and the arguments. We can capture the reference to this returned function by assigning it to a variable and later we can call it any time.

function add(inc1, inc2)
{
    return this.a + inc1 + inc2;
}

var o = { a : 4 };
document.write(add.call(o, 5, 6)+"<br />"); //15
      //above add.call(o,5,6) sets `this` inside
      //add() to `o` and calls add() resulting:
      // this.a + inc1 + inc2 = 
      // `o.a` i.e. 4 + 5 + 6 = 15
document.write(add.apply(o, [5, 6]) + "<br />"); //15
      // `o.a` i.e. 4 + 5 + 6 = 15

var g = add.bind(o, 5, 6);       //g: `o.a` i.e. 4 + 5 + 6
document.write(g()+"<br />");    //15

var h = add.bind(o, 5);          //h: `o.a` i.e. 4 + 5 + ?
document.write(h(6) + "<br />"); //15
      // 4 + 5 + 6 = 15
document.write(h() + "<br />");  //NaN
      //no parameter is passed to h()
      //thus inc2 inside add() is `undefined`
      //4 + 5 + undefined = NaN</code>

7. this inside event handlers

• When you assign function directly to event handlers of an element, use of this directly inside event handling function refers to the corresponding element. Such direct function assignment can be done using addeventListener method or through the traditional event registration methods like onclick.
• Similarly, when you use this directly inside the event property (like <button onclick="...this..." >) of the element, it refers to the element.
• However use of this indirectly through the other function called inside the event handling function or event property resolves to the global object window.
• The same above behavior is achieved when we attach the function to the event handler using Microsoft's Event Registration model method attachEvent. Instead of assigning the function to the event handler (and the thus making the function method of the element), it calls the function on the event (effectively calling it in global context).

I recommend to better try this in JSFiddle.

<script> 
    function clickedMe() {
       alert(this + " : " + this.tagName + " : " + this.id);
    } 
    document.getElementById("button1").addEventListener("click", clickedMe, false);
    document.getElementById("button2").onclick = clickedMe;
    document.getElementById("button5").attachEvent('onclick', clickedMe);   
</script>

<h3>Using `this` "directly" inside event handler or event property</h3>
<button id="button1">click() "assigned" using addEventListner() </button><br />
<button id="button2">click() "assigned" using click() </button><br />
<button id="button3" onclick="alert(this+ ' : ' + this.tagName + ' : ' + this.id);">used `this` directly in click event property</button>

<h3>Using `this` "indirectly" inside event handler or event property</h3>
<button onclick="alert((function(){return this + ' : ' + this.tagName + ' : ' + this.id;})());">`this` used indirectly, inside function <br /> defined & called inside event property</button><br />

<button id="button4" onclick="clickedMe()">`this` used indirectly, inside function <br /> called inside event property</button> <br />

IE only: <button id="button5">click() "attached" using attachEvent() </button>

8. this in ES6 arrow function

In an arrow function, this will behave like common variables: it will be inherited from its lexical scope. The function's this, where the arrow function is defined, will be the arrow function's this.

So, that's the same behavior as:

(function(){}).bind(this)

See the following code:

const globalArrowFunction = () => {
  return this;
};

console.log(globalArrowFunction()); //window

const contextObject = {
  method1: () => {return this},
  method2: function(){
    return () => {return this};
  }
};

console.log(contextObject.method1()); //window

const contextLessFunction = contextObject.method1;

console.log(contextLessFunction()); //window

console.log(contextObject.method2()()) //contextObject

const innerArrowFunction = contextObject.method2();

console.log(innerArrowFunction()); //contextObject 

---

Javascript's this

Simple function invocation

Consider the following function:

function foo() {
    console.log("bar");
    console.log(this);
}
foo(); // calling the function

Note that we are running this in the normal mode, i.e. strict mode is not used.

When running in a browser, the value of this would be logged as window. This is because window is the global variable in a web browser's scope.

If you run this same piece of code in an environment like node.js, this would refer to the global variable in your app.

Now if we run this in strict mode by adding the statement "use strict"; to the beginning of the function declaration, this would no longer refer to the global variable in either of the environments. This is done to avoid confusions in strict mode. this would, in this case just log undefined, because that is what it is, it is not defined.

In the following cases, we would see how to manipulate the value of this.

Calling a function on an object

There are different ways to do this. If you have called native methods in Javascript like forEach and slice, you should already know that the this variable in that case refers to the Object on which you called that function (Note that in javascript, just about everything is an Object, including Arrays and Functions). Take the following code for example.

var myObj = {key: "Obj"};
myObj.logThis = function () {
    // I am a method
    console.log(this);
}
myObj.logThis(); // myObj is logged

If an Object contains a property which holds a Function, the property is called a method. This method, when called, will always have it's this variable set to the Object it is associated with. This is true for both strict and non-strict modes.

Note that if a method is stored (or rather, copied) in another variable, the reference to this is no longer preserved in the new variable. For example:

// continuing with the previous code snippet

var myVar = myObj.logThis;
myVar();
// logs either of window/global/undefined based on mode of operation

Considering a more commonly practical scenario:

var el = document.getElementById('idOfEl');
el.addEventListener('click', function() { console.log(this) });
// the function called by addEventListener contains this as the reference to the element
// so clicking on our element would log that element itself

The new keyword

Consider a constructor function in Javascript:

function Person (name) {
    this.name = name;
    this.sayHello = function () {
        console.log ("Hello", this);
    }
}

var awal = new Person("Awal");
awal.sayHello();
// In `awal.sayHello`, `this` contains the reference to the variable `awal`

How does this work? Well, let's see what happens when we use the new keyword.

1. Calling the function with the new keyword would immediately initialize an Object of type Person.
2. The constructor of this Object has its constructor set to Person. Also, note that typeof awal would return Object only.
3. This new Object would be assigned the prototype of Person.prototype. This means that any method or property in the Person prototype would be available to all instances of Person, including awal.
4. The function Person itself is now invoked; this being a reference to the newly constructed object awal.

Pretty straightforward, eh?

Note that the official ECMAScript spec nowhere states that such types of functions are actual constructor functions. They are just normal functions, and new can be used on any function. It's just that we use them as such, and so we call them as such only.

Calling functions on Functions: call and apply

So yeah, since functions are also Objects (and in-fact first class variables in Javascript), even functions have methods which are... well, functions themselves.

All functions inherit from the global Function, and two of its many methods are call and apply, and both can be used to manipulate the value of this in the function on which they are called.

function foo () { console.log (this, arguments); }
var thisArg = {myObj: "is cool"};
foo.call(thisArg, 1, 2, 3);

This is a typical example of using call. It basically takes the first parameter and sets this in the function foo as a reference to thisArg. All other parameters passed to call is passed to the function foo as arguments.
So the above code will log {myObj: "is cool"}, [1, 2, 3] in the console. Pretty nice way to change the value of this in any function.

apply is almost the same as call accept that it takes only two parameters: thisArg and an array which contains the arguments to be passed to the function. So the above call call can be translated to apply like this:

foo.apply(thisArg, [1,2,3])

Note that call and apply can override the value of this set by dot method invocation we discussed in the second bullet. Simple enough :)

Presenting.... bind!

bind is a brother of call and apply. It is also a method inherited by all functions from the global Function constructor in Javascript. The difference between bind and call/apply is that both call and apply will actually invoke the function. bind, on the other hand, returns a new function with the thisArg and arguments pre-set. Let's take an example to better understand this:

function foo (a, b) {
    console.log (this, arguments);
}
var thisArg = {myObj: "even more cool now"};
var bound = foo.bind(thisArg, 1, 2);
console.log (typeof bound); // logs `function`
console.log (bound);
/* logs `function () { native code }` */

bound(); // calling the function returned by `.bind`
// logs `{myObj: "even more cool now"}, [1, 2]`

See the difference between the three? It is subtle, but they are used differently. Like call and apply, bind will also over-ride the value of this set by dot-method invocation.

Also note that neither of these three functions do any change to the original function. call and apply would return the value from freshly constructed functions while bind will return the freshly constructed function itself, ready to be called.

Extra stuff, copy this

Sometimes, you don't like the fact that this changes with scope, especially nested scope. Take a look at the following example.

var myObj = {
    hello: function () {
        return "world"
        },
    myMethod: function () {
        // copy this, variable names are case-sensitive
        var that = this;
        // callbacks ftw \o/
        foo.bar("args", function () {
            // I want to call `hello` here
            this.hello(); // error
            // but `this` references to `foo` damn!
            // oh wait we have a backup \o/
            that.hello(); // "world"
        });
    }
  };

In the above code, we see that the value of this changed with the nested scope, but we wanted the value of this from the original scope. So we 'copied' this to that and used the copy instead of this. Clever, eh?

Index:

1. What is held in this by default?
2. What if we call the function as a method with Object-dot notation?
3. What if we use the new keyword?
4. How do we manipulate this with call and apply?
5. Using bind.
6. Copying this to solve nested-scope issues.

---

"this" is all about scope. Every function has its own scope, and since everything in JS is an object, even a function can store some values into itself using "this". OOP 101 teaches that "this" is only applicable to instances of an object. Therefore, every-time a function executes, a new "instance" of that function has a new meaning of "this".

Most people get confused when they try to use "this" inside of anonymous closure functions like:

(function(value) {
    this.value = value;
    $('.some-elements').each(function(elt){
        elt.innerHTML = this.value;        // uh oh!! possibly undefined
    });
})(2);

So here, inside each(), "this" doesn't hold the "value" that you expect it to (from

this.value = value;

above it). So, to get over this (no pun intended) problem, a developer could:

(function(value) {
    var self = this;            // small change
    self.value = value;
    $('.some-elements').each(function(elt){
        elt.innerHTML = self.value;        // phew!! == 2 
    });
})(2);

Try it out; you'll begin to like this pattern of programming

---

Since this thread has bumped up, I have compiled few points for readers new to this topic.

How is the value of this determined?

We use this similar to the way we use pronouns in natural languages like English: “John is running fast because he is trying to catch the train.” Instead we could have written “… John is trying to catch the train”.

var person = {    
    firstName: "Penelope",
    lastName: "Barrymore",
    fullName: function () {

    // We use "this" just as in the sentence above:
       console.log(this.firstName + " " + this.lastName);

    // We could have also written:
       console.log(person.firstName + " " + person.lastName);
    }
}

this is not assigned a value until an object invokes the function where it is defined. In the global scope, all global variables and functions are defined on the window object. Therefore, this in a global function refers to (and has the value of) the global window object.

When use strict, this in global and in anonymous functions that are not bound to any object holds a value of undefined.

The this keyword is most misunderstood when: 1) we borrow a method that uses this, 2) we assign a method that uses this to a variable, 3) a function that uses this is passed as a callback function, and 4) this is used inside a closure — an inner function. ⁽²⁾

What holds the future

Defined in ECMA Script 6, arrow-functions adopt the this binding from the enclosing (function or global) scope.

function foo() {
     // return an arrow function
     return (a) => {
     // `this` here is lexically inherited from `foo()`
     console.log(this.a);
  };
}
var obj1 = { a: 2 };
var obj2 = { a: 3 };

var bar = foo.call(obj1);
bar.call( obj2 ); // 2, not 3!

While arrow-functions provide an alternative to using bind(), it’s important to note that they essentially are disabling the traditional this mechanism in favor of more widely understood lexical scoping. ⁽¹⁾

---

References:

1. this & Object Prototypes, by Kyle Simpson. © 2014 Getify Solutions.
2. javascriptissexy.com - http://goo.gl/pvl0GX
3. Angus Croll - http://goo.gl/Z2RacU

---

this in JavaScript always refers to the 'owner' of the function that is being executed.

If no explicit owner is defined, then the top most owner, the window object, is referenced.

So if I did

function someKindOfFunction() {
   this.style = 'foo';
}

element.onclick = someKindOfFunction;

this would refer to the element object. But be careful, a lot of people make this mistake.

<element onclick="someKindOfFunction()">

In the latter case, you merely reference the function, not hand it over to the element. Therefore, this will refer to the window object.

---

Every execution context in javascript has a this parameter that is set by:

1. How the function is called (including as an object method, use of call and apply, use of new)
2. Use of bind
3. Lexically for arrow functions (they adopt the this of their outer execution context)
4. Whether the code is in strict or non-strict mode
5. Whether the code was invoked using eval

You can set the value of this using func.call, func.apply or func.bind.

By default, and what confuses most beginners, when a listener is called after an event is raised on a DOM element, the this value of the function is the DOM element.

jQuery makes this trivial to change with jQuery.proxy.

---

Daniel, awesome explanation! A couple of words on this and good list of this execution context pointer in case of event handlers.

In two words, this in JavaScript points the object from whom (or from whose execution context) the current function was run and it's always read-only, you can't set it anyway (such an attempt will end up with 'Invalid left-hand side in assignment' message.

For event handlers: inline event handlers, such as <element onclick="foo">, override any other handlers attached earlier and before, so be careful and it's better to stay off of inline event delegation at all. And thanks to Zara Alaverdyan who inspired me to this list of examples through a dissenting debate :)

• el.onclick = foo; // in the foo - obj
• el.onclick = function () {this.style.color = '#fff';} // obj
• el.onclick = function() {doSomething();} // In the doSomething - Window
• el.addEventListener('click',foo,false) // in the foo - obj
• el.attachEvent('onclick, function () { // this }') // window, all the compliance to IE :)
• <button onclick="this.style.color = '#fff';"> // obj
• <button onclick="foo"> // In the foo - window, but you can <button onclick="foo(this)">

---

Here is one good source of this in JavaScript.

Here is the summary:

• global this

  In a browser, at the global scope, this is the windowobject

  <script type="text/javascript">
    console.log(this === window); // true
    var foo = "bar";
    console.log(this.foo); // "bar"
    console.log(window.foo); // "bar"
  

  In node using the repl, this is the top namespace. You can refer to it as global.

  >this
    { ArrayBuffer: [Function: ArrayBuffer],
      Int8Array: { [Function: Int8Array] BYTES_PER_ELEMENT: 1 },
      Uint8Array: { [Function: Uint8Array] BYTES_PER_ELEMENT: 1 },
      ...
  >global === this
   true
  

  In node executing from a script, this at the global scope starts as an empty object. It is not the same as global

  \\test.js
  console.log(this);  \\ {}
  console.log(this === global); \\ fasle
  

• function this

Except in the case of DOM event handlers or when a thisArg is provided (see further down), both in node and in a browser using this in a function that is not called with new references the global scope…

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    testThis();
    console.log(this.foo); //logs "foo"
</script>

If you use use strict;, in which case this will be undefined

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      "use strict";
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    testThis();  //Uncaught TypeError: Cannot set property 'foo' of undefined 
</script>

If you call a function with new the this will be a new context, it will not reference the global this.

<script type="text/javascript">
    foo = "bar";

    function testThis() {
      this.foo = "foo";
    }

    console.log(this.foo); //logs "bar"
    new testThis();
    console.log(this.foo); //logs "bar"

    console.log(new testThis().foo); //logs "foo"
</script>

• prototype this

Functions you create become function objects. They automatically get a special prototype property, which is something you can assign values to. When you create an instance by calling your function with new you get access to the values you assigned to the prototype property. You access those values using this.

function Thing() {
  console.log(this.foo);
}

Thing.prototype.foo = "bar";

var thing = new Thing(); //logs "bar"
console.log(thing.foo);  //logs "bar"

It is usually a mistake to assign arrays or objects on the prototype. If you want instances to each have their own arrays, create them in the function, not the prototype.

function Thing() {
    this.things = [];
}

var thing1 = new Thing();
var thing2 = new Thing();
thing1.things.push("foo");
console.log(thing1.things); //logs ["foo"]
console.log(thing2.things); //logs []

• object this

You can use this in any function on an object to refer to other properties on that object. This is not the same as an instance created with new.

var obj = {
    foo: "bar",
    logFoo: function () {
        console.log(this.foo);
    }
};

obj.logFoo(); //logs "bar"

• DOM event this

In an HTML DOM event handler, this is always a reference to the DOM element the event was attached to

function Listener() {
    document.getElementById("foo").addEventListener("click",
       this.handleClick);
}
Listener.prototype.handleClick = function (event) {
    console.log(this); //logs "<div id="foo"></div>"
}

var listener = new Listener();
document.getElementById("foo").click();

Unless you bind the context

function Listener() {
    document.getElementById("foo").addEventListener("click", 
        this.handleClick.bind(this));
}
Listener.prototype.handleClick = function (event) {
    console.log(this); //logs Listener {handleClick: function}
}

var listener = new Listener();
document.getElementById("foo").click();

• HTML this

Inside HTML attributes in which you can put JavaScript, this is a reference to the element.

<div id="foo" onclick="console.log(this);"></div>
<script type="text/javascript">
document.getElementById("foo").click(); //logs <div id="foo"...
</script>

• eval this

You can use eval to access this.

function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
    eval("console.log(this.foo)"); //logs "bar"
}

var thing = new Thing();
thing.logFoo();

• with this

You can use with to add this to the current scope to read and write to values on this without referring to this explicitly.

function Thing () {
}
Thing.prototype.foo = "bar";
Thing.prototype.logFoo = function () {
    with (this) {
        console.log(foo);
        foo = "foo";
    }
}

var thing = new Thing();
thing.logFoo(); // logs "bar"
console.log(thing.foo); // logs "foo"

• jQuery this

the jQuery will in many places have this refer to a DOM element.

<div class="foo bar1"></div>
<div class="foo bar2"></div>
<script type="text/javascript">
$(".foo").each(function () {
    console.log(this); //logs <div class="foo...
});
$(".foo").on("click", function () {
    console.log(this); //logs <div class="foo...
});
$(".foo").each(function () {
    this.click();
});
</script>

---

There is a lot of confusion regarding how "this" keyword is interpreted in JavaScript. Hopefully this article will lay all those to rest once and for all. And a lot more. Please read the entire article carefully. Be forewarned that this article is long.

Irrespective of the context in which it is used, "this" always references the "current object" in Javascript. However, what the "current object" is differs according to context. The context may be exactly 1 of the 6 following:

1. Global (i.e. Outside all functions)
2. Inside Direct "Non Bound Function" Call (i.e. a function that has not been bound by calling functionName.bind)
3. Inside Indirect "Non Bound Function" Call through functionName.call and functionName.apply
4. Inside "Bound Function" Call (i.e. a function that has been bound by calling functionName.bind)
5. While Object Creation through "new"
6. Inside Inline DOM event handler

The following describes each of this contexts one by one:

1. Global Context (i.e. Outside all functions):

   Outside all functions (i.e. in global context) the "current object" (and hence the value of "this") is always the "window" object for browsers.

2. Inside Direct "Non Bound Function" Call:

   Inside a Direct "Non Bound Function" Call, the object that invoked the function call becomes the "current object" (and hence the value of "this"). If a function is called without a explicit current object, the current object is either the "window" object (For Non Strict Mode) or undefined (For Strict Mode) . Any function (or variable) defined in Global Context automatically becomes a property of the "window" object.For e.g Suppose function is defined in Global Context as

   function UserDefinedFunction(){
       alert(this)
       }
   

   it becomes the property of the window object, as if you have defined it as

   window.UserDefinedFunction=function(){
     alert(this)
   }  
   

   In "Non Strict Mode", Calling/Invoking this function directly through "UserDefinedFunction()" will automatically call/invoke it as "window.UserDefinedFunction()" making "window" as the "current object" (and hence the value of "this") within "UserDefinedFunction".Invoking this function in "Non Strict Mode" will result in the following

   UserDefinedFunction() // displays [object Window]  as it automatically gets invoked as window.UserDefinedFunction()
   

   In "Strict Mode", Calling/Invoking the function directly through "UserDefinedFunction()" will "NOT" automatically call/invoke it as "window.UserDefinedFunction()".Hence the "current object" (and the value of "this") within "UserDefinedFunction" shall be undefined. Invoking this function in "Strict Mode" will result in the following

   UserDefinedFunction() // displays undefined
   

   However, invoking it explicitly using window object shall result in the following

   window.UserDefinedFunction() // "always displays [object Window]   irrespective of mode."
   

   Let us look at another example. Please look at the following code

    function UserDefinedFunction()
       {
           alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
       }
   
   var o1={
               a:1,
               b:2,
               f:UserDefinedFunction
         }
   var o2={
               c:3,
               d:4,
               f:UserDefinedFunction
          }
   
   o1.f() // Shall display 1,2,undefined,undefined
   o2.f() // Shall display undefined,undefined,3,4
   

   In the above example we see that when "UserDefinedFunction" was invoked through o1, "this" takes value of o1 and the value of its properties "a" and "b" get displayed. The value of "c" and "d" were shown as undefined as o1 does not define these properties

   Similarly when "UserDefinedFunction" was invoked through o2, "this" takes value of o2 and the value of its properties "c" and "d" get displayed.The value of "a" and "b" were shown as undefined as o2 does not define these properties.

3. Inside Indirect "Non Bound Function" Call through functionName.call and functionName.apply:

   When a "Non Bound Function" is called through functionName.call or functionName.apply, the "current object" (and hence the value of "this") is set to the value of "this" parameter (first parameter) passed to call/apply. The following code demonstrates the same.

   function UserDefinedFunction()
   {
       alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
   }
   var o1={
               a:1,
               b:2,
               f:UserDefinedFunction
          }
   var o2={
               c:3,
               d:4,
               f:UserDefinedFunction
          }
   
   UserDefinedFunction.call(o1) // Shall display 1,2,undefined,undefined
   UserDefinedFunction.apply(o1) // Shall display 1,2,undefined,undefined
   
   UserDefinedFunction.call(o2) // Shall display undefined,undefined,3,4
   UserDefinedFunction.apply(o2) // Shall display undefined,undefined,3,4
   
   o1.f.call(o2) // Shall display undefined,undefined,3,4
   o1.f.apply(o2) // Shall display undefined,undefined,3,4
   
   o2.f.call(o1) // Shall display 1,2,undefined,undefined
   o2.f.apply(o1) // Shall display 1,2,undefined,undefined
   

   The above code clearly shows that the "this" value for any "NON Bound Function" can be altered through call/apply. Also,if the "this" parameter is not explicitly passed to call/apply, "current object" (and hence the value of "this") is set to "window" in Non strict mode and "undefined" in strict mode.

4. Inside "Bound Function" Call (i.e. a function that has been bound by calling functionName.bind):

   A bound function is a function whose "this" value has been fixed. The following code demonstrated how "this" works in case of bound function

   function UserDefinedFunction()
   {
       alert(this.a + ","  + this.b + ","  + this.c  + ","  + this.d)
   }
   var o1={
             a:1,
             b:2,
             f:UserDefinedFunction,
             bf:null
          }
   var o2={
              c:3,
              d:4,
              f:UserDefinedFunction,
              bf:null
           }
   
   var bound1=UserDefinedFunction.bind(o1); // permanantly fixes "this" value of function "bound1" to Object o1
   bound1() // Shall display 1,2,undefined,undefined
   
   var bound2=UserDefinedFunction.bind(o2); // permanantly fixes "this" value of function "bound2" to Object o2
   bound2() // Shall display undefined,undefined,3,4
   
   var bound3=o1.f.bind(o2); // permanantly fixes "this" value of function "bound3" to Object o2
   bound3() // Shall display undefined,undefined,3,4
   
   var bound4=o2.f.bind(o1); // permanantly fixes "this" value of function "bound4" to Object o1
   bound4() // Shall display 1,2,undefined,undefined
   
   o1.bf=UserDefinedFunction.bind(o2) // permanantly fixes "this" value of function "o1.bf" to Object o2
   o1.bf() // Shall display undefined,undefined,3,4
   
   o2.bf=UserDefinedFunction.bind(o1) // permanantly fixes "this" value of function "o2.bf" to Object o1
   o2.bf() // Shall display 1,2,undefined,undefined
   
   bound1.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
   
   bound1.apply(o2) // Shall still display 1,2,undefined,undefined. "apply" cannot alter the value of "this" for bound function
   
   o2.bf.call(o2) // Shall still display 1,2,undefined,undefined. "call" cannot alter the value of "this" for bound function
   o2.bf.apply(o2) // Shall still display 1,2,undefined,undefined."apply" cannot alter the value of "this" for bound function
   

   As given in the code above, "this" value for any "Bound Function" CANNOT be altered through call/apply. Also, if the "this" parameter is not explicitly passed to bind, "current object" (and hence the value of "this" ) is set to "window" in Non strict mode and "undefined" in strict mode. One more thing. Binding an already bound function does not change the value of "this". It remains set as the value set by first bind function.

5. While Object Creation through "new":

   Inside a constructor function, the "current object" (and hence the value of "this") references the object that is currently being created through "new" irrespective of the bind status of the function. However if the constructor is a bound function it shall get called with predefined set of arguments as set for the bound function.

6. Inside Inline DOM event handler:

   Please look at the following HTML Snippet

   <button onclick='this.style.color=white'>Hello World</button>
   <div style='width:100px;height:100px;' onclick='OnDivClick(event,this)'>Hello World</div>
   

   The "this" in above examples refer to "button" element and the "div" element respectively.

   In the first example, the font color of the button shall be set to white when it is clicked.

   In the second example when the "div" element is clicked it shall call the OnDivClick function with its second parameter referencing the clicked div element. However the value of "this" within OnDivClick SHALL NOT reference the clicked div element. It shall be set as the "window object" or "undefined" in Non strict and Strict Modes respectively (if OnDivClick is an unbound function) or set to a predefined Bound value (if OnDivClick is a bound function)

The following summarizes the entire article

1. In Global Context "this" always refers to the "window" object

2. Whenever a function is invoked, it is invoked in context of an object ("current object"). If the current object is not explicitly provided, the current object is the "window object" in NON Strict Mode and "undefined" in Strict Mode by default.

3. The value of "this" within a Non Bound function is the reference to object in context of which the function is invoked ("current object")

4. The value of "this" within a Non Bound function can be overriden by call and apply methods of the function.

5. The value of "this" is fixed for a Bound function and cannot be overriden by call and apply methods of the function.

6. Binding and already bound function does not change the value of "this". It remains set as the value set by first bind function.

7. The value of "this" within a constructor is the object that is being created and initialized

8. The value of "this" within an inline DOM event handler is reference to the element for which the event handler is given.

---

Probably the most detailed and comprehensive article on this is the following:

Gentle explanation of 'this' keyword in JavaScript

The idea behind this is to understand that the function invocation types have the significant importance on setting this value.

---

When having troubles identifying this, do not ask yourself:

Where is this taken from?

but do ask yourself:

How is the function invoked?

For an arrow function (special case of context transparency) ask yourself:

What value has this where the arrow function is defined?

This mindset is correct when dealing with this and will save you from headache.

---

This is the best explanation I've seen: Understand JavaScripts this with Clarity

The this reference ALWAYS refers to (and holds the value of) an object—a singular object—and it is usually used inside a function or a method, although it can be used outside a function in the global scope. Note that when we use strict mode, this holds the value of undefined in global functions and in anonymous functions that are not bound to any object.

There are Four Scenarios where this can be confusing:

1. When we pass a method (that uses this) as an argument to be used as a callback function.
2. When we use an inner function (a closure). It is important to take note that closures cannot access the outer function’s this variable by using the this keyword because the this variable is accessible only by the function itself, not by inner functions.
3. When a method which relies on this is assigned to a variable across contexts, in which case this references another object than originally intended.
4. When using this along with the bind, apply, and call methods.

He gives code examples, explanations, and solutions, which I thought was very helpful.

---

this is one of the misunderstood concept in JavaScript because it behaves little differently from place to place. Simply, this refers to the "owner" of the function we are currently executing.

this helps to get the current object (a.k.a. execution context) we work with. If you understand in which object the current function is getting executed, you can understand easily what current this is

var val = "window.val"

var obj = {
    val: "obj.val",
    innerMethod: function () {
        var val = "obj.val.inner",
            func = function () {
                var self = this;
                return self.val;
            };

        return func;
    },
    outerMethod: function(){
        return this.val;
    }
};

//This actually gets executed inside window object 
console.log(obj.innerMethod()()); //returns window.val

//Breakdown in to 2 lines explains this in detail
var _inn = obj.innerMethod();
console.log(_inn()); //returns window.val

console.log(obj.outerMethod()); //returns obj.val

Above we create 3 variables with same name 'val'. One in global context, one inside obj and the other inside innerMethod of obj. JavaScript resolves identifiers within a particular context by going up the scope chain from local go global.

---

Few places where this can be differentiated

Calling a method of a object

var status = 1;
var helper = {
    status : 2,
    getStatus: function () {
        return this.status;
    }
};

var theStatus1 = helper.getStatus(); //line1
console.log(theStatus1); //2

var theStatus2 = helper.getStatus;
console.log(theStatus2()); //1

When line1 is executed, JavaScript establishes an execution context (EC) for the function call, setting this to the object referenced by whatever came before the last ".". so in the last line you can understand that a() was executed in the global context which is the window.

With Constructor

this can be used to refer to the object being created

function Person(name){
    this.personName = name;
    this.sayHello = function(){
        return "Hello " + this.personName;
    }
}

var person1 = new Person('Scott');
console.log(person1.sayHello()); //Hello Scott

var person2 = new Person('Hugh');
var sayHelloP2 = person2.sayHello;
console.log(sayHelloP2()); //Hello undefined

When new Person() is executed, a completely new object is created. Person is called and its this is set to reference that new object.

Function call

function testFunc() {
    this.name = "Name";
    this.myCustomAttribute = "Custom Attribute";
    return this;
}

var whatIsThis = testFunc();
console.log(whatIsThis); //window

var whatIsThis2 = new testFunc();
console.log(whatIsThis2);  //testFunc() / object

console.log(window.myCustomAttribute); //Custom Attribute 

If we miss new keyword, whatIsThis referes to the most global context it can find(window)

With event handlers

If the event handler is inline, this refers to global object

<script type="application/javascript">
    function click_handler() {
        alert(this); // alerts the window object
    }
</script>

<button id='thebutton' onclick='click_handler()'>Click me!</button>

When adding event handler through JavaScript, this refers to DOM element that generated the event.

---

• You can also manipulate the context using .apply() .call() and .bind()
• JQuery proxy is another way you can use to make sure this in a function will be the value you desire. (Check Understanding $.proxy(), jQuery.proxy() usage)
• What does var that = this means in JavaScript

---

The value of "this" depends on the "context" in which the function is executed. The context can be any object or the global object, i.e., window.

So the Semantic of "this" is different from the traditional OOP languages. And it causes problems: 1. when a function is passed to another variable (most likely, a callback); and 2. when a closure is invoked from a member method of a class.

In both cases, this is set to window.

---

In pseudoclassical terms, the way many lectures teach the 'this' keyword is as an object instantiated by a class or object constructor. Each time a new object is constructed from a class, imagine that under the hood a local instance of a 'this' object is created and returned. I remember it taught like this:

function Car(make, model, year) {
var this = {}; // under the hood, so to speak
this.make = make;
this.model = model;
this.year = year;
return this; // under the hood
}

var mycar = new Car('Eagle', 'Talon TSi', 1993);
// ========= under the hood
var this = {};
this.make = 'Eagle';
this.model = 'Talon TSi';
this.year = 1993;
return this;

---

Whould this help? (Most confusion of 'this' in javascript is coming from the fact that it generally is not linked to your object, but to the current executing scope -- that might not be exactly how it works but is always feels like that to me -- see the article for a complete explanation)

---

A little bit info about this keyword

Let's log this keyword to the console in global scope without any more code but

console.log(this)

In Client/Browser this keyword is a global object which is window

console.log(this === window) // true

and

In Server/Node/Javascript runtime this keyword is also a global object which is module.exports

console.log(this === module.exports) // true
console.log(this === exports) // true

Keep in mind exports is just a reference to module.exports

---

I have a different take on this from the other answers that I hope is helpful.

One way to look at JavaScript is to see that there are only 1 way to call a function¹. It is

functionObject.call(objectForThis, arg0, arg1, arg2, ...);

There is always some value supplied for objectForThis.

Everything else is syntactic sugar for functionObject.call

So, everything else can be described by how it translates into functionObject.call.

If you just call a function then this is the "global object" which in the browser is the window

function foo() {
  console.log(this);
}

foo();  // this is the window object

In other words,

foo();

was effectively translated into

foo.call(window);

Note that if you use strict mode then this will be undefined

'use strict';

function foo() {
  console.log(this);
}

foo();  // this is the window object

which means

In other words,

foo();

was effectively translated into

foo.call(undefined);

In JavaScript there are operators like + and - and *. There is also the dot operator which is .

The . operator when used with a function on the right and an object on the left effectively means "pass object as this to function.

Example

const bar = {
  name: 'bar',
  foo() { 
    console.log(this); 
  },
};

bar.foo();  // this is bar

In other words bar.foo() translates into const temp = bar.foo; temp.call(bar);

Note that it doesn't matter how the function was created (mostly...). All of these will produce the same results

const bar = {
  name: 'bar',
  fn1() { console.log(this); },
  fn2: function() { console.log(this); },
  fn3: otherFunction,
};

function otherFunction() { console.log(this) };

bar.fn1();  // this is bar
bar.fn2();  // this is bar
bar.fn3();  // this is bar

Again these all are just syntactic sugar for

{ const temp = bar.fn1; temp.call(bar); }
{ const temp = bar.fn2; temp.call(bar); }
{ const temp = bar.fn3; temp.call(bar); }

One other wrinkle is the prototype chain. When you use a.b JavaScript first looks on the object referenced directly by a for the property b. If b is not found on the object then JavaScript will look in the object's prototype to find b.

There are various ways to define an object's prototype, the most common in 2019 is the class keyword. For the purposes of this though it doesn't matter. What matters is that as it looks in object a for property b if it finds property b on the object or in it's prototype chain if b ends up being a function then the same rules as above apply. The function b references will be called using the call method and passing a as objectForThis as shown a the top of this answer.

Now. Let's imagine we make a function that explicitly sets this before calling another function and then call it with the . (dot) operator

function foo() {
  console.log(this);
}

function bar() {
  const objectForThis = {name: 'moo'}
  foo.call(objectForThis);  // explicitly passing objectForThis
}

const obj = {
  bar,
};

obj.bar();  

Following the translation to use call, obj.bar() becomes const temp = obj.bar; temp.call(obj);. When we enter the bar function we call foo but we explicitly passed in another object for objectForThis so when we arrive at foo this is that inner object.

This is what both bind and => functions effectively do. They are more syntactic sugar. They effectively build a new invisible function exactly like bar above that explicitly sets this before it calls whatever function is specified. In the case of bind this is set to whatever you pass to bind.

function foo() {
  console.log(this);
}

const bar = foo.bind({name: 'moo'});

// bind created a new invisible function that calls foo with the bound object.

bar();  

// the objectForThis we are passing to bar here is ignored because
// the invisible function that bind created will call foo with with
// the object we bound above

bar.call({name: 'other'});

Note that if functionObject.bind did not exist we could make our own like this

function bind(fn, objectForThis) {
  return function(...args) {
    return fn.call(objectForthis, ...args);
  };
}

and then we could call it like this

function foo() {
  console.log(this);
}

const bar = bind(foo, {name:'abc'});

Arrow functions, the => operator are syntactic sugar for bind

const a = () => {console.log(this)};

is the same as

const tempFn = function() {console.log(this)}; 
const a = tempFn.bind(this);

Just like bind, a new invisible function is created that calls the given function with a bound value for objectForThis but unlike bind the object to be bound is implicit. It's whatever this happens to be when the => operator is used.

So, just like the rules above

const a = () => { console.log(this); }  // this is the global object

'use strict';
const a = () => { console.log(this); }  // this is undefined

function foo() {
  return () => { console.log(this); }
}

const obj = {
  foo,
};
const b = obj.foo();
b();

obj.foo() translates to const temp = obj.foo; temp.call(obj); which means the arrow operator inside foo will bind obj to a new invisible function and return that new invisible function which is assigned to b. b() will work as it always has as b.call(window) or b.call(undefined) calling the new invisible function that foo created. That invisible function ignores the this passed into it and passes obj as objectForThis` to the arrow function.

The code above translates to

function foo() {
  function tempFn() {
    console.log(this);
  }
  return tempFn.bind(this);
}

const obj = {
  foo,
};
const b = obj.foo();
b.call(window or undefined if strict mode);

¹apply is another function similar to call

functionName.apply(objectForThis, arrayOfArgs);

But as of ES6 conceptually you can even translate that into

functionName.call(objectForThis, ...arrayOfArgs);

---

this use for Scope just like this

  <script type="text/javascript" language="javascript">
$('#tbleName tbody tr').each(function{
var txt='';
txt += $(this).find("td").eq(0).text();
\\same as above but synatx different
var txt1='';
 txt1+=$('#tbleName tbody tr').eq(0).text();
alert(txt1)
});
</script>

value of txt1 and txt is same in Above example $(this)=$('#tbleName tbody tr') is Same

---

Summary this Javascript:

• The value of this is determined by how the function is invoked not, where it was created!
• Usually the value of this is determined by the Object which is left of the dot. (window in global space)
• In event listeners the value of this refers to the DOM element on which the event was called.
• When in function is called with the new keyword the value of this refers to the newly created object
• You can manipulate the value of this with the functions: call, apply, bind

Example:

let object = {
  prop1: function () {console.log(this);}
}

object.prop1();   // object is left of the dot, thus this is object

const myFunction = object.prop1 // We store the function in the variable myFunction

myFunction(); // Here we are in the global space
              // myFunction is a property on the global object
              // Therefore it logs the window object
              
             

Example event listeners:

document.querySelector('.foo').addEventListener('click', function () {
  console.log(this);   // This refers to the DOM element the eventListener was invoked from
})


document.querySelector('.foo').addEventListener('click', () => {
  console.log(this);  // Tip, es6 arrow function don't have their own binding to the this v
})                    // Therefore this will log the global object

.foo:hover {
  color: red;
  cursor: pointer;
}

<div class="foo">click me</div>

Example constructor:

function Person (name) {
  this.name = name;
}

const me = new Person('Willem');
// When using the new keyword the this in the constructor function will refer to the newly created object

console.log(me.name); 
// Therefore, the name property was placed on the object created with new keyword.

---

"this" in JavaScript this is one of the properties of the Execution Context.

• this property is created every time a function is executed and not before that.
• Its value is not static but rather depends on how it is being used.
• takes a value that points to the owner of the function in which it is used

There are different ways in which the "this" keyword can be used, below are the example for it (method, regular function, arrow function, Event listener, Explicit function Binding).

1. Inside a method.

this === (to the object that is calling the Method).

In the above example the method " fullName()" is called by an Object "person" hence the value of this inside the method " fullName()" will be equal to the "person" Object.

2. Inside a Function.

i) function declaration/expression

in loose mode this === window (object)

in Strict mode this === undefined

Note : this property works the same while defining a function using function declaration or function expression approach.

ii) Arrow Function :

Arrow Function does not have their own this property, they take the value of this as their surrounding Function. If the surrounding function is not present i.e if they are defined at the global level then this === window (object)

3. Event Listener this === object on which the handler is attached. click event bind to the Document object

In the above example since the click handler is attached to the "document" object, this will be equal to the "document" object

4. Explicit Function Binding (call, Apply, Bind)

The call() and apply() methods are predefined JavaScript methods.

They can both be used to call an object method with another object as an argument.

In the above example this inside the "printFullDetails()" is explicitly set to the personObj1 and personObj2 by passing as the first argument to call method.

You can Explore more about call, apply and bind methods here.

---

To understand "this" properly one must understand the context and scope and difference between them.

Scope: In javascript scope is related to the visibility of the variables, scope achieves through the use of the function. (Read more about scope)

Context: Context is related to objects. It refers to the object to which a function belongs. When you use the JavaScript “this” keyword, it refers to the object to which function belongs. For example, inside of a function, when you say: “this.accoutNumber”, you are referring to the property “accoutNumber”, that belongs to the object to which that function belongs.

If the object “myObj” has a method called “getMyName”, when the JavaScript keyword “this” is used inside of “getMyName”, it refers to “myObj”. If the function “getMyName” were executed in the global scope, then “this” refers to the window object (except in strict mode).

Now let's see some example:

    <script>
        console.log('What is this: '+this);
        console.log(this);
    </script>

Runnig abobve code in browser output will:

According to the output you are inside of the context of the window object, it is also visible that window prototype refers to the Object.

Now let's try inside of a function:

    <script>
        function myFunc(){
            console.log('What is this: '+this);
            console.log(this);
        }
        myFunc();
    </script>

Output:

The output is the same because we logged 'this' variable in the global scope and we logged it in functional scope, we didn't change the context. In both case context was same, related to widow object.

Now let's create our own object. In javascript, you can create an object in many ways.

 <script>
        var firstName = "Nora";
        var lastName = "Zaman";
        var myObj = {
            firstName:"Lord",
            lastName:'Baron',
            printNameGetContext:function(){
                console.log(firstName + " "+lastName);
                console.log(this.firstName +" "+this.lastName);
                return this;
            }
        }

      var context = myObj.printNameGetContext();
      console.log(context);
    </script>

Output:

So from the above example, we found that 'this' keyword is referring to a new context that is related to myObj, and myObject also has prototype chain to Object.

Let's go throw another example:

<body>
    <button class="btn">Click Me</button>
    <script>
        function printMe(){
            //Terminal2: this function declared inside window context so this function belongs to the window object.
            console.log(this);
        }
        document.querySelector('.btn').addEventListener('click', function(){
            //Terminal1: button context, this callback function belongs to DOM element 
            console.log(this);
            printMe();
        })
    </script>
</body>

output: Make sense right? (read comments)

If you having trouble to understand the above example let's try with our own callback;

<script>
        var myObj = {
            firstName:"Lord",
            lastName:'Baron',
            printName:function(callback1, callback2){
                //Attaching callback1 with this myObj context
                this.callback1 = callback1;
                this.callback1(this.firstName +" "+this.lastName)
                //We did not attached callback2 with myObj so, it's reamin with window context by default
                callback2();
                /*
                 //test bellow codes
                 this.callback2 = callback2;
                 this.callback2();
                */
            }
        }
        var callback2 = function (){
            console.log(this);
        }
        myObj.printName(function(data){
            console.log(data);
            console.log(this);
        }, callback2);
    </script>

output:

Now let's Understand Scope, Self, IIFE and THIS how behaves

       var color = 'red'; // property of window
       var obj = {
           color:'blue', // property of window
           printColor: function(){ // property of obj, attached with obj
               var self = this;
               console.log('In printColor -- this.color: '+this.color);
               console.log('In printColor -- self.color: '+self.color);
               (function(){ // decleard inside of printColor but not property of object, it will executed on window context.
                    console.log(this)
                    console.log('In IIFE -- this.color: '+this.color);
                    console.log('In IIFE -- self.color: '+self.color); 
               })();

               function nestedFunc(){// decleard inside of printColor but not property of object, it will executed on window context.
                    console.log('nested fun -- this.color: '+this.color);
                    console.log('nested fun -- self.color: '+self.color);
               }

               nestedFunc(); // executed on window context
               return nestedFunc;
           }
       };

       obj.printColor()(); // returned function executed on window context
   </script> 

Output is pretty awesome right?