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

This is an example of a very rare occasion where it is recommended to use == instead of ===. Expression somevar == null will return true for undefined and null, but false for everything else (an error if variable is undeclared).

Using the != will flip the result, as expected.

Modern editors will not warn for using == or != operator with null, as this is almost always the desired behavior.

Most common comparisions:

undeffinedVar == null     // true
obj.undefinedProp == null // true
null == null              // true
0 == null                 // false
'0' == null               // false
'' == null                // false

Try it yourself:

let undefinedVar;
console.table([
    { test : undefinedVar,     result: undefinedVar     == null },
    { test : {}.undefinedProp, result: {}.undefinedProp == null },
    { test : null,             result: null             == null },
    { test : false,            result: false            == null },
    { test : 0,                result: 0                == null },
    { test : '',               result: ''               == null },
    { test : '0',              result: '0'              == null },
]);