variable - javascript null vs undefined




How to determine if variable is 'undefined' or 'null'? (17)

How do I determine if variable 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.


if(x==null) is bad idea in javascript,judge with "==" may cause unexpected type coercion, and can't be read by coffee-script, never use "==" or "!=" in condition judgment!

if(x) will be better.but becareful 0 and "", it will be treat as false,not equal method with "!= null" is true.

https://www.w3schools.com/js/js_best_practices.asp


Best way:

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

/* do your stuff */
}

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.


I see that the original post was using JQuery. So I am adding what I have used in the past via JQuery. Usually when I am performing such a check I am not necessarily checking for the value to be null or undefined just for that purpose. More like "Is the value there so I can perform actions on it.". So for that I use the following. I have not tried every type but I know it works for both null and undefined. Also, objects and arrays(empty version of those will return true). Also, of course if the variable does not exist you will get an exception but I honestly try to avoid that at all costs. So I agree with Aerovistae. I want to know about this in my code and don't want to just skip over it.

if ($.trim(value) != '') {
   //Do Something
}

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

if (object) { Somecode}

i.e.

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

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.


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;

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.

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


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

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

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

You can check it by this method-

if(['object','undefined'].indexOf(typeof(variable)) != -1){
    console.log("variable is null or undefined.");
}

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.


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 }

jQuery check element not null

var dvElement = $('#dvElement');

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

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

will evaluate to true if value is not:

  • null

  • undefined

  • NaN

  • empty string ("")

  • 0

  • false


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

var x;
if (x === undefined) {
    alert ("only declared, but not defined.")
};
if (typeof y === "undefined") {
    alert ("not even declared.")
};

You can only use second one: as it will check for both definition and declaration





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