Checking for `null` or `undefined` in JavaScript

javascript-null-undefined.md

Checking for null or undefined in JavaScript

Say I want to check if a value v is either null or undefined in JavaScript. I might use this conditional:

if (!v) { ... } // 🙅‍♀️ wrong!

Because of JavaScript's weak typing, this conditional will catch way more than just null and undefined.

Instead, use this:

if (v == undefined) { ... } // ✅ right!

This uses the weakly-typed == to our advantage. According to the ECMAScript spec, x == y is true when x is null and y is undefined; trivially, it's also true when x and y are the exact same value (e.g.,. both x and y are undefined). This behaviour is explicit in the definition of ==, so this is one of the only times I prefer to use == over the far stricter (and safer) === comparison.

Truth table

I've listed here a bunch of different values of v and the results of !v and v == undefined:

v	desired	!v	v == undefined
undefined	true	true	true
null	true	true	true
0	false	true	false
false	false	true	false
''	false	true	false
NaN	false	true	false