Coder Perfect

In JavaScript, how do you check for a variable that isn’t defined?


I needed to know if the variable was defined or not. The following, for example, results in a not-defined error.

alert( x );

What is the best way for me to catch this error?

Asked by Jineesh

Solution #1

Null is an object in JavaScript. There’s an undefined value for items that don’t exist. The DOM returns null in almost all circumstances where it can’t detect a structure in the document, whereas undefined is the value used in JavaScript.

Second, no, there isn’t an exact match. If you absolutely want to check for null, follow these steps:

if (yourvar === null) // Does not execute if yourvar is `undefined`

Try/catch is the only way to check if a variable exists, because typeof treats an undeclared variable and a variable declared with the value undefined as identical.

However, to see if a variable is declared and not undefined, type:

if (yourvar !== undefined) // Any scope

Because undefined may be reassigned like a variable, it was previously required to use the typeof operator to check for it safely. The former method was as follows:

if (typeof yourvar !== 'undefined') // Any scope

In ECMAScript 5, which was released in 2009, the issue of undefined being re-assignable was resolved. Because undefined has been read-only for some time, you may now safely use === and!== to test for it without requiring typeof.

If you just want to know if a member exists but don’t care about its value, use this formula:

if ('membername' in object) // With inheritance
if (object.hasOwnProperty('membername')) // Without inheritance

If you want to know if a variable is true or not, use the following formula:

if (yourvar)


Answered by Natrium

Solution #2

The following is the only technique to genuinely test if a variable is undefined. Remember that in JavaScript, undefined is an object.

if (typeof someVar === 'undefined') {
  // Your variable is undefined

Some of the other solutions in this thread will lead you to believe a variable is undefined even though it has been defined (with a value of NULL or 0, for instance).

Answered by Michael Wales

Solution #3

Technically, I believe the correct solution is:

typeof x === "undefined"

It’s easy to become complacent and rely on shortcuts.

x == null

However, an undefined variable x, as well as a variable x containing null, can both yield true.

Answered by Jason S

Solution #4

A simpler and more condensed version would be:

if (!x) {


if (typeof x !== "undefined") {
    //Do something since x is defined.

Answered by Dmitri Farkov

Solution #5

I’ve often done:

function doSomething(variable)
    var undef;

    if(variable === undef)
         alert('Hey moron, define this bad boy.');

Answered by Joe

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