How to check for an undefined or null variable in JavaScript?

Question

We are frequently using the following code pattern in our JavaScript code

if (typeof(some_variable) != \'undefined\' && some_variable != null)
{
             


        

Answer 1

Since there is no single complete and correct answer, I will try to summarize:

In general, the expression:

if (typeof(variable) != "undefined" && variable != null)

cannot be simplified, because the variable might be undeclared so omitting the typeof(variable) != "undefined" would result in ReferenceError. But, you can simplify the expression according to the context:

If the variable is global, you can simplify to:

if (window.variable != null)

If it is local, you can probably avoid situations when this variable is undeclared, and also simplify to:

if (variable != null)

If it is object property, you don't have to worry about ReferenceError:

if (obj.property != null)

Answer 2

Open the Developer tools in your browser and just try the code shown in the below image.

Answer 3

Checking null with normal equality will also return true for undefined.

if (window.variable == null) alert('variable is null or undefined');

Answer 4

You can just check if the variable has a value or not. Meaning,

if( myVariable ) {
//mayVariable is not :
//null
//undefined
//NaN
//empty string ("")
//0
//false

}

If you do not know whether a variable exists (that means, if it was declared) you should check with the typeof operator. e.g.

if( typeof myVariable !== 'undefined' ) {
    // myVariable will get resolved and it is defined
}

Answer 5

You must define a function of this form:

validate = function(some_variable){
    return(typeof(some_variable) != 'undefined' && some_variable != null)
}

Answer 6

In ES5 or ES6 if you need check it several times you cand do:

const excluded = [null, undefined, ''];

if (!exluded.includes(varToCheck) {
  // it will bee not null, not undefined and not void string
}