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Generating random numbers with vanilla JS

Today, we’re going to look at how to create random numbers with vanilla JavaScript.

The Math.random() method

The Math.random() method generates a random float (a number with decimals) between 0 and 1.

// Logs something like this: 0.37111265461165543
// It will be different every time
var rand = Math.random();

The number the Math.random() method generates is inclusive of 0 (as in, it could sometimes be 0, though I’ve never personally seen that happen), but exclusive of 1 (as in, it will never reach 1).

Here’s a demo.

Getting numbers bigger than 0

What if you wanted to get integers, or whole numbers, instead of floats?

To do that, we can multiply whatever Math.random() returns by a number that’s a power of ten. The bigger the number you multiple by, the more digits in the number.

var randomOverZero = function (pow) {
	return Math.random() * pow;

// Logs something like: 14.48985072988853
var rand100 = randomOverZero(100);

// Logs something like: 9005.187977724258
var rand10000 = randomOverZero(10000);

Here’s another demo.

Getting a random integer

What if you wanted a random integer, or whole number, instead of a float?

For that, we can use the Math.floor() method after multiplying the returned value of Math.random() by our power of ten.

var randomInteger = function (pow) {
	return Math.floor(Math.random() * pow);

// Logs something like: 14
var rand100 = randomInteger(100);

// Logs something like: 9005
var rand10000 = randomInteger(10000);

Here’s a demo of the random integer technique.

Getting a random integer between two numbers

Finally, let’s look at how to get a random integer between two numbers. For example, let’s say you wanted a number that was at least 5, but no bigger than 42.

For that, we’ll create a helper function that accepts a min and max for the random number as arguments.

Then, we’ll subtract the min from the max, and add 1 to it (otherwise the max would never be reached). We’ll multiply the returned value of Math.random() by this new number, and add the min to it. This gives us a random float between our two values.

Finally, we’ll use Math.floor() to turn it into an integer, and return the result.

var randomNumber = function (min, max) {
	return Math.floor(Math.random() * (max - min + 1) + min);

// Logs something like 37
var rand = randomNumber(5, 42);

Props to the MDN examples section for Math.random() for helping me figure this one out.

You can play with a demo on CodePen, or download this on the Vanilla JS Toolkit.