How can I determine if a variable is 'undefined' or 'null'?_问答_开发者

How do I determine if variable开发者_运维知识库 is undefined or null?

My code is as follows:

var EmpName = $("div#esd-names div#name").attr('class');
if(EmpName == 'undefined'){
  // DO SOMETHING
};

<div id="esd-names">
  <div id="name"></div>
</div>

But if I do this, the JavaScript interpreter halts execution.

You can use the qualities of the abstract equality operator to do this:

if (variable == null){
    // your code here.
}

Because null == undefined is true, the above code will catch both null and undefined.

The standard way to catch null and undefined simultaneously is this:

if (variable == null) {
     // do something 
}

--which is 100% equivalent to the more explicit but less concise:

if (variable === undefined || variable === null) {
     // do something 
}

When writing professional JS, it's taken for granted that type equality and the behavior of == vs === is understood. Therefore we use == and only compare to null.

Edit again

The comments suggesting the use of typeof are simply wrong. Yes, my solution above will cause a ReferenceError if the variable doesn't exist. This is a good thing. This ReferenceError is desirable: it will help you find your mistakes and fix them before you ship your code, just like compiler errors would in other languages. Use try/catch if you are working with input you don't have control over.

You should not have any references to undeclared variables in your code.

Combining the above answers, it seems the most complete answer would be:

if( typeof variable === 'undefined' || variable === null ){
    // Do stuff
}

This should work for any variable that is either undeclared or declared and explicitly set to null or undefined. The boolean expression should evaluate to false for any declared variable that has an actual non-null value.

if (variable == null) {
    // Do stuff, will only match null or undefined, this won't match false
}

if (typeof EmpName != 'undefined' && EmpName) {

will evaluate to true if value is not:

null
undefined
NaN
empty string ("")
0
false

Probably the shortest way to do this is:

if(EmpName == null) { /* DO SOMETHING */ };

Here is proof:

function check(EmpName) {
  if(EmpName == null) { return true; };
  return false;
}

var log = (t,a) => console.log(`${t} -> ${check(a)}`);

log('null', null);
log('undefined', undefined);
log('NaN', NaN);
log('""', "");
log('{}', {});
log('[]', []);
log('[1]', [1]);
log('[0]', [0]);
log('[[]]', [[]]);
log('true', true);
log('false', false);
log('"true"', "true");
log('"false"', "false");
log('Infinity', Infinity);
log('-Infinity', -Infinity);
log('1', 1);
log('0', 0);
log('-1', -1);
log('"1"', "1");
log('"0"', "0");
log('"-1"', "-1");

// "void 0" case
console.log('---\n"true" is:', true);
console.log('"void 0" is:', void 0);
log(void 0,void 0); // "void 0" is "undefined"

And here are more details about == (source here)

How can I determine if a variable is 'undefined' or 'null'?

BONUS: reason why === is more clear than == (look on agc answer)

How can I determine if a variable is 'undefined' or 'null'?

jQuery attr() function returns either a blank string or the actual value (and never null or undefined). The only time it returns undefined is when your selector didn't return any element.

So you may want to test against a blank string. Alternatively, since blank strings, null and undefined are false-y, you can just do this:

if (!EmpName) { //do something }

Edited answer: In my opinion, you shouldn't use the function from my below old answer. Instead, you should probably know the type of your variable and use the according to check directly (for example, wondering if an array is empty? just do if(arr.length===0){} etc.). This answer doesn't even answer OP's question.

I've come to write my own function for this. JavaScript is weird.

It is usable on literally anything. (Note that this also checks if the variable contains any usable values. But since this information is usually also needed, I think it's worth posting). Please consider leaving a note.

function empty(v) {
    let type = typeof v;
    if (type === 'undefined') {
        return true;
    }
    if (type === 'boolean') {
        return !v;
    }
    if (v === null) {
        return true;
    }
    if (v === undefined) {
        return true;
    }
    if (v instanceof Array) {
        if (v.length < 1) {
            return true;
        }
    } else if (type === 'string') {
        if (v.length < 1) {
            return true;
        }
        if (v === '0') {
            return true;
        }
    } else if (type === 'object') {
        if (Object.keys(v).length < 1) {
            return true;
        }
    } else if (type === 'number') {
        if (v === 0) {
            return true;
        }
    }
    return false;
}

TypeScript-compatible.

This function should do exactly the same thing like PHP's empty() function (see RETURN VALUES)

Considers undefined, null, false, 0, 0.0, "0" {}, [] as empty.

"0.0", NaN, " ", true are considered non-empty.

The shortest and easiest:

if(!EmpName ){
 // DO SOMETHING
}

this will evaluate true if EmpName is:

null
undefined
NaN
empty
string ("")
0
false

If the variable you want to check is a global, do

if (window.yourVarName) {
    // Your code here
}

This way to check will not throw an error even if the yourVarName variable doesn't exist.

Example: I want to know if my browser supports History API

if (window.history) {
    history.back();
}

How this works:

window is an object which holds all global variables as its properties, and in JavaScript it is legal to try to access a non-existing object property. If history doesn't exist then window.history returns undefined. undefined is falsey, so code in an if(undefined){} block won't run.

In JavaScript, as per my knowledge, we can check an undefined, null or empty variable like below.

if (variable === undefined){
}

if (variable === null){
}

if (variable === ''){
}

Check all conditions:

if(variable === undefined || variable === null || variable === ''){
}

Since you are using jQuery, you can determine whether a variable is undefined or its value is null by using a single function.

var s; // undefined
jQuery.isEmptyObject(s); // will return true;

s = null; // defined as null
jQuery.isEmptyObject(s); // will return true;

// usage
if(jQuery.isEmptyObject(s)){
    alert('Either variable: s is undefined or its value is null');
}else{
     alert('variable: s has value ' + s);
}

s = 'something'; // defined with some value
jQuery.isEmptyObject(s); // will return false;

I've just had this problem i.e. checking if an object is null.
I simply use this:

if (object) {
    // Your code
}

For example:

if (document.getElementById("enterJob")) {
    document.getElementById("enterJob").className += ' current';
}

You can simply use the following (I know there are shorter ways to do this, but this may make it easier to visually observe, at least for others looking at the code).

if (x === null || x === undefined) {
 // Add your response code here, etc.
}

source: https://www.growthsnippets.com/how-can-i-determine-if-a-variable-is-undefined-or-null/

jQuery check element not null:

var dvElement = $('#dvElement');

if (dvElement.length  > 0) {
    // Do something
}
else{
    // Else do something else
}

With the newest javascript changes, you can use the new logical operator ??= to check if the left operand is null or undefined and if so assign the value of right operand.

SO,

if(EmpName == null){  // if Variable EmpName null or undefined
  EmpName = 'some value';
};

Is equivalent to:

EmpName ??= 'some value';

The easiest way to check is:

if(!variable) {
  // If the variable is null or undefined then execution of code will enter here.
}

I run this test in the Chrome console. Using (void 0) you can check undefined:

var c;
undefined
if (c === void 0) alert();
// output =  undefined
var c = 1;
// output =  undefined
if (c === void 0) alert();
// output =   undefined
// check c value  c
// output =  1
if (c === void 0) alert();
// output =  undefined
c = undefined;
// output =  undefined
if (c === void 0) alert();
// output =   undefined

With the solution below:

const getType = (val) => typeof val === 'undefined' || !val ? null : typeof val;
const isDeepEqual = (a, b) => getType(a) === getType(b);

console.log(isDeepEqual(1, 1)); // true
console.log(isDeepEqual(null, null)); // true
console.log(isDeepEqual([], [])); // true
console.log(isDeepEqual(1, "1")); // false
etc...

I'm able to check for the following:

null
undefined
NaN
empty
string ("")
0
false

To test if a variable is null or undefined I use the below code.

    if(typeof sVal === 'undefined' || sVal === null || sVal === ''){
      console.log('variable is undefined or null');
    }

if you create a function to check it:

export function isEmpty (v) {
 if (typeof v === "undefined") {
   return true;
 }
 if (v === null) {
   return true;
 }
 if (typeof v === "object" && Object.keys(v).length === 0) {
   return true;
 }

 if (Array.isArray(v) && v.length === 0) {
   return true;
 }

 if (typeof v === "string" && v.trim().length === 0) {
   return true;
 }

return false;
}

(null == undefined)  // true

(null === undefined) // false

Because === checks for both the type and value. Type of both are different but value is the same.

Let's look at this,

```
let apple; // Only declare the variable as apple
alert(apple); // undefined
```
In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.

   let apple = null; /* Declare the variable as apple and initialized but the value is null */
   alert(apple); // null

In the second one it displays null, because variable of apple value is null.

So you can check whether a value is undefined or null.

if(apple !== undefined || apple !== null) {
    // Can use variable without any error
}

The foo == null check should do the trick and resolve the "undefined OR null" case in the shortest manner. (Not considering "foo is not declared" case.) But people who are used to have 3 equals (as the best practice) might not accept it. Just look at eqeqeq or triple-equals rules in eslint and tslint...

The explicit approach, when we are checking if a variable is undefined or null separately, should be applied in this case, and my contribution to the topic (27 non-negative answers for now!) is to use void 0 as both short and safe way to perform check for undefined.

Using foo === undefined is not safe because undefined is not a reserved word and can be shadowed (MDN). Using typeof === 'undefined' check is safe, but if we are not going to care about foo-is-undeclared case the following approach can be used:

if (foo === void 0 || foo === null) { ... }

You can do something like this, I think its more efficient for multiple value check on the same variable in one condition

const x = undefined;
const y = null;
const z = 'test';

if ([undefined, null].includes(x)) {
  // Will return true
}

if ([undefined, null].includes(y)) {
  // Will return true
}

if ([undefined, null].includes(z)) {
  // Will return false
}

No one seems to have to posted this yet, so here we go:

a?.valueOf() === undefined works reliably for either null or undefined.

The following works pretty much like a == null or a == undefined, but it could be more attractive for purists who don't like ==