In today's post, I'm going to show you how to get a random truthy/falsy value in JavaScript. This can be helpful when you want to run some conditional statements in a randomized way. It can be done with a really simple helper function:

/**
* Get a random truthy/falsy value
* {@link https://gist.github.com/kieranbarker/25b1b4b4cef14c65b10dcefdc462f6ee}
* @returns {Number} 1 (truthy) or 0 (falsy)
*/

function fiftyFifty () {
return Math.round(Math.random());
}

You would use it like so:

if (fiftyFifty()) {
// This will run 50% of the time...
}

Let's see how it works.

Math.random()

The first thing we do is make a call to the static Math.random() method. This returns a pseudo-random number between 0 (inclusive) and 1 (exclusive). This means that it could return 0, or anything between 0 and 1, but not 1.

Here are some examples. I just ran Math.random() in the console five times:

  • 0.8424333633138914
  • 0.6708640956019702
  • 0.29226591753427367
  • 0.9309103559864624
  • 0.9709222745438715

Math.round()

Once we've got our random number, we pass it to the static Math.round() method. This does what you'd expect: it returns the number after rounding it to the nearest integer. If the fractional part of the number is less than 0.5, the number will be rounded down. If it's greater than or equal to 0.5, the number will be rounded up.

Because Math.random() always returns a number between 0 and 1, our helper function will always return 0 (rounded down) or 1 (rounded up).

Truthiness and falsiness

In most (if not all) programming languages, a value is either truthy or falsy. Because 1 is truthy and 0 is falsy, this means you can call our helper function and use the returned value as a condition, as I demonstrated before:

if (fiftyFifty()) {
// This will run 50% of the time...
}

Returning an explicit boolean value

It's not necessary, but if you really wanted to return an explicit boolean value for some reason, you could force the conversion using a double NOT (!) operator:

/**
* Get a random boolean value
* {@link https://gist.github.com/kieranbarker/25b1b4b4cef14c65b10dcefdc462f6ee}
* @returns {Boolean} True or false
*/

function fiftyFifty () {
return !!Math.round(Math.random());
}

The MDN Web Docs explain:

The logical NOT (!) operator ... takes truth to falsity and vice versa. It is typically used with Boolean (logical) values. When used with non-Boolean values, it returns false if its single operand can be converted to true; otherwise, returns true.

If Math.round(Math.random()) returns 0, then the first NOT (!) operator will convert it to true, since !0 is true. The second NOT (!) operator then turns it back to false, since !true is false.

If Math.round(Math.random()) returns 1, then the first NOT (!) operator will convert it to false, since !1 is false. The second NOT (!) operator then turns it back to true, since !false is true.


I first saw Steve Griffith use the Math.round(Math.random()) technique in one of his videos called Efficient ES6 async await with fetch, so credit to him for that.

If you have any questions, please email me at kieranbarker@outlook.com!