How do you generate a random number in javascript?

Try it

Note: 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

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 reach 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 [and not equal] max.

function getRandomArbitrary[min, max] {
  return 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]; // The maximum is exclusive and the minimum is inclusive
}

Note: It might be tempting to use Math.round[] 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]; // The maximum is inclusive and the minimum is inclusive
}

See also