What is the difference between using Function.prototype.apply() and Function.prototype.call() to invoke a function?

var func = function() {
  alert('hello!');
};

func.apply(); vs func.call();

Are there performance differences between the two aforemention开发者_StackOverflowed methods? When is it best to use call over apply and vice versa?

The difference is that apply lets you invoke the function with arguments as an array; call requires the parameters be listed explicitly. A useful mnemonic is "A for array and C for comma."

See MDN's documentation on apply and call.

Pseudo syntax:

theFunction.apply(valueForThis, arrayOfArgs)

theFunction.call(valueForThis, arg1, arg2, ...)

There is also, as of ES6, the possibility to spread the array for use with the call function, you can see the compatibilities here.

Sample code:

function theFunction(name, profession) {
    console.log("My name is " + name + " and I am a " + profession +".");
}
theFunction("John", "fireman");
theFunction.apply(undefined, ["Susan", "school teacher"]);
theFunction.call(undefined, "Claude", "mathematician");
theFunction.call(undefined, ...["Matthew", "physicist"]); // used with the spread operator

K. Scott Allen has a nice writeup on the matter.

Basically, they differ on how they handle function arguments.

The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method."

So:

// assuming you have f
function f(message) { ... }
f.call(receiver, "test");
f.apply(receiver, ["test"]);

To answer the part about when to use each function, use apply if you don't know the number of arguments you will be passing, or if they are already in an array or array-like object (like the arguments object to forward your own arguments. Use call otherwise, since there's no need to wrap the arguments in an array.

f.call(thisObject, a, b, c); // Fixed number of arguments

f.apply(thisObject, arguments); // Forward this function's arguments

var args = [];
while (...) {
    args.push(some_value());
}
f.apply(thisObject, args); // Unknown number of arguments

When I'm not passing any arguments (like your example), I prefer call since I'm calling the function. apply would imply you are applying the function to the (non-existent) arguments.

There shouldn't be any performance differences, except maybe if you use apply and wrap the arguments in an array (e.g. f.apply(thisObject, [a, b, c]) instead of f.call(thisObject, a, b, c)). I haven't tested it, so there could be differences, but it would be very browser specific. It's likely that call is faster if you don't already have the arguments in an array and apply is faster if you do.

Here's a good mnemonic. Apply uses Arrays and Always takes one or two Arguments. When you use Call you have to Count the number of arguments.

While this is an old topic, I just wanted to point out that .call is slightly faster than .apply. I can't tell you exactly why.

See jsPerf, http://jsperf.com/test-call-vs-apply/3

[UPDATE!]

Douglas Crockford mentions briefly the difference between the two, which may help explain the performance difference... http://youtu.be/ya4UHuXNygM?t=15m52s

Apply takes an array of arguments, while Call takes zero or more individual parameters! Ah hah!

.apply(this, [...])

.call(this, param1, param2, param3, param4...)

Follows an extract from Closure: The Definitive Guide by Michael Bolin. It might look a bit lengthy, but it's saturated with a lot of insight. From "Appendix B. Frequently Misunderstood JavaScript Concepts":

What `this` Refers to When a Function is Called

When calling a function of the form foo.bar.baz(), the object foo.bar is referred to as the receiver. When the function is called, it is the receiver that is used as the value for this:

var obj = {};
obj.value = 10;
/** @param {...number} additionalValues */
obj.addValues = function(additionalValues) {
  for (var i = 0; i < arguments.length; i++) {
    this.value += arguments[i];
  }
  return this.value;
};
// Evaluates to 30 because obj is used as the value for 'this' when
// obj.addValues() is called, so obj.value becomes 10 + 20.
obj.addValues(20);

If there is no explicit receiver when a function is called, then the global object becomes the receiver. As explained in "goog.global" on page 47, window is the global object when JavaScript is executed in a web browser. This leads to some surprising behavior:

var f = obj.addValues;
// Evaluates to NaN because window is used as the value for 'this' when
// f() is called. Because and window.value is undefined, adding a number to
// it results in NaN.
f(20);
// This also has the unintentional side effect of adding a value to window:
alert(window.value); // Alerts NaN

Even though obj.addValues and f refer to the same function, they behave differently when called because the value of the receiver is different in each call. For this reason, when calling a function that refers to this, it is important to ensure that this will have the correct value when it is called. To be clear, if this were not referenced in the function body, then the behavior of f(20) and obj.addValues(20) would be the same.

Because functions are first-class objects in JavaScript, they can have their own methods. All functions have the methods call() and apply() which make it possible to redefine the receiver (i.e., the object that this refers to) when calling the function. The method signatures are as follows:

/**
* @param {*=} receiver to substitute for 'this'
* @param {...} parameters to use as arguments to the function
*/
Function.prototype.call;
/**
* @param {*=} receiver to substitute for 'this'
* @param {Array} parameters to use as arguments to the function
*/
Function.prototype.apply;

Note that the only difference between call() and apply() is that call() receives the function parameters as individual arguments, whereas apply() receives them as a single array:

// When f is called with obj as its receiver, it behaves the same as calling
// obj.addValues(). Both of the following increase obj.value by 60:
f.call(obj, 10, 20, 30);
f.apply(obj, [10, 20, 30]);

The following calls are equivalent, as f and obj.addValues refer to the same function:

obj.addValues.call(obj, 10, 20, 30);
obj.addValues.apply(obj, [10, 20, 30]);

However, since neither call() nor apply() uses the value of its own receiver to substitute for the receiver argument when it is unspecified, the following will not work:

// Both statements evaluate to NaN
obj.addValues.call(undefined, 10, 20, 30);
obj.addValues.apply(undefined, [10, 20, 30]);

The value of this can never be null or undefined when a function is called. When null or undefined is supplied as the receiver to call() or apply(), the global object is used as the value for receiver instead. Therefore, the previous code has the same undesirable side effect of adding a property named value to the global object.

It may be helpful to think of a function as having no knowledge of the variable to which it is assigned. This helps reinforce the idea that the value of this will be bound when the function is called rather than when it is defined.

End of extract.

It is useful at times for one object to borrow the function of another object, meaning that the borrowing object simply executes the lent function as if it were its own.

A small code example:

var friend = {
    car: false,
    lendCar: function ( canLend ){
      this.car = canLend;
 }

}; 

var me = {
    car: false,
    gotCar: function(){
      return this.car === true;
  }
};

console.log(me.gotCar()); // false

friend.lendCar.call(me, true); 

console.log(me.gotCar()); // true

friend.lendCar.apply(me, [false]);

console.log(me.gotCar()); // false

These methods are very useful for giving objects temporary functionality.

Another example with Call, Apply and Bind. The difference between Call and Apply is evident, but Bind works like this:

Bind returns an instance of a function that can be executed
First Parameter is 'this'
Second parameter is a Comma separated list of arguments (like Call)

}

function Person(name) {
    this.name = name; 
}
Person.prototype.getName = function(a,b) { 
     return this.name + " " + a + " " + b; 
}

var reader = new Person('John Smith');

reader.getName = function() {
   // Apply and Call executes the function and returns value

   // Also notice the different ways of extracting 'getName' prototype
   var baseName = Object.getPrototypeOf(this).getName.apply(this,["is a", "boy"]);
   console.log("Apply: " + baseName);

   var baseName = Object.getPrototypeOf(reader).getName.call(this, "is a", "boy"); 
   console.log("Call: " + baseName);

   // Bind returns function which can be invoked
   var baseName = Person.prototype.getName.bind(this, "is a", "boy"); 
   console.log("Bind: " + baseName());
}

reader.getName();
/* Output
Apply: John Smith is a boy
Call: John Smith is a boy
Bind: John Smith is a boy
*/

I'd like to show an example, where the 'valueForThis' argument is used:

Array.prototype.push = function(element) {
   /*
   Native code*, that uses 'this'       
   this.put(element);
   */
}
var array = [];
array.push(1);
array.push.apply(array,[2,3]);
Array.prototype.push.apply(array,[4,5]);
array.push.call(array,6,7);
Array.prototype.push.call(array,8,9);
//[1, 2, 3, 4, 5, 6, 7, 8, 9]

**details: http://es5.github.io/#x15.4.4.7*

Call() takes comma-separated arguments, ex:

.call(scope, arg1, arg2, arg3)

and apply() takes an array of arguments, ex:

.apply(scope, [arg1, arg2, arg3])

here are few more usage examples: http://blog.i-evaluation.com/2012/08/15/javascript-call-and-apply/

From the MDN docs on Function.prototype.apply() :

The apply() method calls a function with a given this value and arguments provided as an array (or an array-like object).

Syntax
fun.apply(thisArg, [argsArray])

From the MDN docs on Function.prototype.call() :

The call() method calls a function with a given this value and arguments provided individually.

Syntax
fun.call(thisArg[, arg1[, arg2[, ...]]])

From Function.apply and Function.call in JavaScript :

The apply() method is identical to call(), except apply() requires an array as the second parameter. The array represents the arguments for the target method.

Code example :

var doSomething = function() {
    var arr = [];
    for(i in arguments) {
        if(typeof this[arguments[i]] !== 'undefined') {
            arr.push(this[arguments[i]]);
        }
    }
    return arr;
}

var output = function(position, obj) {
    document.body.innerHTML += '<h3>output ' + position + '</h3>' + JSON.stringify(obj) + '\n<br>\n<br><hr>';
}

output(1, doSomething(
    'one',
    'two',
    'two',
    'one'
));

output(2, doSomething.apply({one : 'Steven', two : 'Jane'}, [
    'one',
    'two',
    'two',
    'one'
]));

output(3, doSomething.call({one : 'Steven', two : 'Jane'},
    'one',
    'two',
    'two',
    'one'
));

Summary:

Both call() and apply() are methods which are located on Function.prototype. Therefore they are available on every function object via the prototype chain. Both call() and apply() can execute a function with a specified value of the this.

The main difference between call() and apply() is the way you have to pass in arguments into it. In both call() and apply() you pass as a first argument the object you want to be the value as this. The other arguments differ in the following way:

With call() you have to put in the arguments normally (starting from the second argument)
With apply() you have to pass in array of arguments.

Example:

let obj = {
  val1: 5,
  val2: 10
}

const summation = function (val3, val4) {
  return  this.val1 + this.val2 + val3 + val4;
}

console.log(summation.apply(obj, [2 ,3]));
// first we assign we value of this in the first arg
// with apply we have to pass in an array


console.log(summation.call(obj, 2, 3));
// with call we can pass in each arg individually

Why would I need to use these functions?

The this value can be tricky sometimes in javascript. The value of this determined when a function is executed not when a function is defined. If our function is dependend on a right this binding we can use call() and apply() to enforce this behaviour. For example:

var name = 'unwantedGlobalName';

const obj =  {
  name: 'Willem',
  sayName () { console.log(this.name);}
}


let copiedMethod = obj.sayName;
// we store the function in the copiedmethod variable



copiedMethod();
// this is now window, unwantedGlobalName gets logged

copiedMethod.call(obj);
// we enforce this to be obj, Willem gets logged

We can differentiate call and apply methods as below

CALL : A function with argument provide individually. If you know the arguments to be passed or there are no argument to pass you can use call.

APPLY : Call a function with argument provided as an array. You can use apply if you don't know how many argument are going to pass to the function.

There is a advantage of using apply over call, we don't need to change the number of argument only we can change a array that is passed.

There is not big difference in performance. But we can say call is bit faster as compare to apply because an array need to evaluate in apply method.

The main difference is, using call, we can change the scope and pass arguments as normal, but apply lets you call it using arguments as an Array (pass them as an array). But in terms of what they to do in your code, they are pretty similar.

While the syntax of this function is almost identical to that of apply(), the fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.

So as you see, there is not a big difference, but still, there are cases we prefer using call() or apply(). For example, look at the code below, which finding the smallest and largest number in an array from MDN, using the apply method:

// min/max number in an array
var numbers = [5, 6, 2, 3, 7];

// using Math.min/Math.max apply
var max = Math.max.apply(null, numbers); 
// This about equal to Math.max(numbers[0], ...)
// or Math.max(5, 6, ...)

var min = Math.min.apply(null, numbers)

So the main difference is just the way we passing the arguments:

Call:

function.call(thisArg, arg1, arg2, ...);

Apply:

function.apply(thisArg, [argsArray]);

Difference between these to methods are, how you want to pass the parameters.

“A for array and C for comma” is a handy mnemonic.

Call and apply both are used to force the this value when a function is executed. The only difference is that call takes n+1 arguments where 1 is this and 'n' arguments. apply takes only two arguments, one is this the other is argument array.

The advantage I see in apply over call is that we can easily delegate a function call to other function without much effort;

function sayHello() {
  console.log(this, arguments);
}

function hello() {
  sayHello.apply(this, arguments);
}

var obj = {name: 'my name'}
hello.call(obj, 'some', 'arguments');

Observe how easily we delegated hello to sayHello using apply, but with call this is very difficult to achieve.

Even though call and apply achive the same thing, I think there is atleast one place where you cannot use call but can only use apply. That is when you want to support inheritance and want to call the constructor.

Here is a function allows you to create classes which also supports creating classes by extending other classes.

function makeClass( properties ) {
    var ctor = properties['constructor'] || function(){}
    var Super = properties['extends'];
    var Class = function () {
                 // Here 'call' cannot work, only 'apply' can!!!
                 if(Super)
                    Super.apply(this,arguments);  
                 ctor.apply(this,arguments);
                }
     if(Super){
        Class.prototype = Object.create( Super.prototype );
        Class.prototype.constructor = Class;
     }
     Object.keys(properties).forEach( function(prop) {
           if(prop!=='constructor' && prop!=='extends')
            Class.prototype[prop] = properties[prop];
     });
   return Class; 
}

//Usage
var Car = makeClass({
             constructor: function(name){
                         this.name=name;
                        },
             yourName: function() {
                     return this.name;
                   }
          });
//We have a Car class now
 var carInstance=new Car('Fiat');
carInstance.youName();// ReturnsFiat

var SuperCar = makeClass({
               constructor: function(ignore,power){
                     this.power=power;
                  },
               extends:Car,
               yourPower: function() {
                    return this.power;
                  }
              });
//We have a SuperCar class now, which is subclass of Car
var superCar=new SuperCar('BMW xy',2.6);
superCar.yourName();//Returns BMW xy
superCar.yourPower();// Returns 2.6

Let me add a little detail to this.

these two calls are almost equivalent:

func.call(context, ...args); // pass an array as list with spread operator

func.apply(context, args);   // is same as using apply

There’s only a minor difference:

The spread operator ... allows passing iterable args as the list to call.

The apply accepts only array-like args.

So, these calls complement each other. Where we expect an iterable, call works, where we expect an array-like, apply works.

And for objects that are both iterable and array-like, like a real array, we technically could use any of them, but apply will probably be faster because most JavaScript engines internally optimize it better.

I just want to add a simple example to a well explained post by flatline, which makes it easy to understand for beginners.

func.call(context, args1, args2 );   // pass arguments as "," separated value

func.apply(context, [args1, args2]); // pass arguments as "Array"

we also use "Call" and "Apply" method for changing reference as defined in code below

let Emp1 = {
  name: 'X',
  getEmpDetail: function(age, department) {
    console.log(`Name: ${this.name}    Age: ${age}    Department: ${department}`)
  }
}

Emp1.getEmpDetail(23, 'Delivery')

// 1st approach of changing "this"
let Emp2 = {
  name: 'Y',
  getEmpDetail: Emp1.getEmpDetail
}

Emp2.getEmpDetail(55, 'Finance')

// 2nd approach of changing "this" using "Call" and "Apply"
let Emp3 = {
  name: 'Emp3_Object',
}

Emp1.getEmpDetail.call(Emp3, 30, 'Admin')

// here we have change the ref from **Emp1 to Emp3**  object
// now this will print "Name =  Emp3_Object" because it is pointing to Emp3 object
Emp1.getEmpDetail.apply(Emp3, [30, 'Admin'])

The call() method calls a function with a given this value and a second parameter which are arguments separated by comma.

object.someMethod.call( someObject, arguments )

The apply() method is the same as call except the fact that the second argument it takes is an array of arguments .

object.someMethod.apply( someObject, arrayOfarguments )

var car  = {  
  name: "Reno",
  country: "France",
  showBuyer: function(firstName, lastName) {
    console.log(`${firstName} ${lastName} just bought a ${this.name} from ${this.country}`);
  }
}

const firstName = "Bryan";
const lastName = "Smith";

car.showBuyer(firstName, lastName);  // Bryan just bought a Reno from France

const obj = { name: "Maserati", country: "Italy" };

car.showBuyer.call(obj, firstName, lastName); // Bryan Smith just bought a Maserati from Italy

car.showBuyer.apply(obj, [firstName, lastName]); // Bryan Smith just bought a Maserati from Italy