How to check a not-defined variable in JavaScript

Question

I wanted to check whether the variable is defined or not. For example, the following throws a not-defined error

alert( x );

How can I catch this error?

Answer 1

An even easier and more shorthand version would be:

if (!x) {
   //Undefined
}

OR

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

Answer 2

I use a small function to verify a variable has been declared, which really cuts down on the amount of clutter in my javascript files. I add a check for the value to make sure that the variable not only exists, but has also been assigned a value. The second condition checks whether the variable has also been instantiated, because if the variable has been defined but not instantiated (see example below), it will still throw an error if you try to reference it's value in your code.

Not instantiated - var my_variable; Instantiated - var my_variable = "";

function varExists(el) { 
  if ( typeof el !== "undefined" && typeof el.val() !== "undefined" ) { 
    return true; 
  } else { 
    return false; 
  } 
}

You can then use a conditional statement to test that the variable has been both defined AND instantiated like this...

if ( varExists(variable_name) ) { // checks that it DOES exist } 

or to test that it hasn't been defined and instantiated use...

if( !varExists(variable_name) ) { // checks that it DOESN'T exist }

Answer 3

The void operator returns undefined for any argument/expression passed to it. so you can test against the result (actually some minifiers change your code from undefined to void 0 to save a couple of characters)

For example:

void 0
// undefined

if (variable === void 0) {
    // variable is undefined
}

Answer 4

I've often done:

function doSomething(variable)
{
    var undef;

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

Answer 5

I often use the simplest way:

var variable;
if (variable === undefined){
    console.log('Variable is undefined');
} else {
    console.log('Variable is defined');
}

EDIT:

Without initializing the variable, exception will be thrown "Uncaught ReferenceError: variable is not defined..."

Answer 6

In JavaScript, null is an object. There's another value for things that don't exist, undefined. The DOM returns null for almost all cases where it fails to find some structure in the document, but in JavaScript itself undefined is the value used.

Second, no, there is not a direct equivalent. If you really want to check for specifically for null, do:

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

If you want to check if a variable exists, that can only be done with try/catch, since typeof will treat an undeclared variable and a variable declared with the value of undefined as equivalent.

But, to check if a variable is declared and is not undefined:

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

If you know the variable exists, and want to check whether there's any value stored in it:

if (yourvar !== undefined)

If you want to know if a member exists independent but don't care what its value is:

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

If you want to to know whether a variable is truthy:

if (yourvar)

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