MDN wants to talk to developers like you: https://qsurvey.mozilla.com/s3/8d22564490d8

The Math.random() function returns a floating-point, pseudo-random number in the range [0, 1); that is, from 0 (inclusive) up to but not including 1 (exclusive), which you can then scale to your desired range. The implementation selects the initial seed to the random number generation algorithm; it cannot be chosen or reset by the user.

Math.random() does not provide cryptographically secure random numbers. Do not use them for anything related to security. Use the Web Crypto API instead, and more precisely the window.crypto.getRandomValues() method.

Syntax

Math.random()

Return value

A floating-point, pseudo-random number between 0 (inclusive) and 1 (exclusive).

Examples

Note that as numbers in JavaScript are IEEE 754 floating point numbers with round-to-nearest-even behavior, the ranges claimed for the functions below (excluding the one for Math.random() itself) aren't exact. If extremely large bounds are chosen (253 or higher), it's possible in extremely rare cases to calculate the usually-excluded upper bound.

Getting a random number between 0 (inclusive) and 1 (exclusive)

function getRandom() {
  return Math.random();
}

Getting a random number between two values

This example returns a random number between the specified values. The returned value is no lower than (and may possibly equal) min, and is less than (but not equal to) max.

function getRandomArbitrary(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

Getting a random integer between two values

This example returns a random integer between the specified values. The value is no lower than min (or the next integer greater than min if min isn't an integer), and is less than (but not equal to) max.

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min)) + min;
}

It might be tempting to use rounding to accomplish that, but doing so would cause your random numbers to follow a non-uniform distribution, which may not be acceptable for your needs.

Getting a random integer between two values, inclusive

While the getRandomInt() function above is inclusive at the minimum, it's exclusive at the maximum. What if you need the results to be inclusive at both the minimum and the maximum? The getRandomIntInclusive() function below accomplishes that.

function getRandomIntInclusive(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. JavaScript 1.0 (UNIX Only) / JavaScript 1.1 (All platforms).
ECMAScript 5.1 (ECMA-262)
The definition of 'Math.random' in that specification.
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Math.random' in that specification.
Standard  
ECMAScript Latest Draft (ECMA-262)
The definition of 'Math.random' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)