Truthiness in JavaScript

In JavaScript, truthiness is whether something returns true or false in an if statement.

if (true) {
	// truthy!
}

if (false) {
	// not truthy...
}

You can check truthiness a bunch of different ways depending on what you’re trying to do. Let’s look at some examples.

Checking explicit values

If you want to check for truthiness, you can check it’s value explicitly.

// Falsey
var isNull = null;
var isUndef = undefined;
var isFalse = false;
var isZero = 0;

// Truthy
var isDef = 'Hello, world!';
var isTrue = true;
var isNotZero = 1;

if (isNull === null) {
	console.log('This will log to the console');
}

if (isUndef === null) {
	console.log('This will not, because it is undefined, not null');
}

if (isfalse === true) {
	console.log('This, of course, will not log either');
}

Simpler `if` statements

But you can also check general truthiness without doing a type comparison.

// These all will NOT log
if (null) {
	console.log('no');
}

if (undefined) {
	console.log('no');
}

if (false) {
	console.log('no');
}

// These all will
if ('Hello world') {
	console.log('yes');
}

if (true) {
	console.log('yes');
}

if (42) {
	console.log('yes');
}

It’s important to keep in mind with these simple checks that 0 is falsey, and 1 is truthy.

// This will NOT log
if (0) {
	console.log('no');
}

// This will
if (1) {
	console.log('yes');
}

This approach is particularly useful when paired with things like querySelector() to check if an element is in the DOM.

var form = document.querySelector('form');
if (form) {
	// Initialize validation script
}

Negative Checks

You can prefix a simple check with a bang (!) to make it a negative check.

// This logs to the console
if (!null) {
	console.log('yep!');
}

Wait, what? null returns falsey, and the bang reverses it to truthy.

You can also prefix items with a double bang (!!) to force them into a strict true/false boolean.

// Logs null
console.log(null);

// Logs false
console.log(!!null);

Which should use and when?

I personally tend to use simpler if statements most of the time so that I don’t have to specify if it’s null, undefined, etc. If I know a number might be returned, though, and that the number could be 0, I’ll use a more specific falsey check.

Checking explicit values

Simpler if statements

Negative Checks

Which should use and when?

Simpler `if` statements