Non-security random integers
Choose an array index or generate a bounded integer for simulations and games.
JavaScript · Function
Math.random
Math.random() returns an approximately uniformly distributed pseudo-random floating-point number from 0 inclusive to 1 exclusive. It accepts no arguments and is not suitable for cryptographic security.
- Since
- July 2015
How it works
Calling Math.random() produces a pseudo-random floating-point value in the interval from 0, inclusive, to 1, exclusive. Multiply it by a range width to scale the interval, then use Math.floor() when an integer is required.
Syntax
Math.random()Returns
A floating-point, pseudo-random number between 0 (inclusive) and 1 (exclusive).
Key points
- The result is a floating-point pseudo-random number in the half-open interval [0, 1).
- Values are approximately uniformly distributed across the supported interval.
- Multiplying by a range width and adding a lower bound transforms the result into a custom range.
- The runtime chooses the generator's initial seed rather than exposing seed control to JavaScript.
Common mistakes
- Do not use this function for passwords, tokens, keys, or other security-sensitive values; use the Web Crypto API instead.
- The seed is implementation-selected, so callers cannot choose or reset it for reproducible sequences.
Example: Generate a random integer
function randomInt(max) {
return Math.floor(Math.random() * max);
}
console.log(randomInt(3)); // 0, 1, or 2Scales the returned value to a range and floors it to produce an integer from 0 through max - 1.
When to use it
Scaled floating-point ranges
Create a fractional value within an application-defined numeric interval.
Approximate randomized behavior
Vary visual effects, test data, or randomized UI behavior where unpredictability is not a security requirement.
Interview questions
QWhat does Math.random() return?
It returns a pseudo-random floating-point number greater than or equal to 0 and less than 1. The distribution is approximately uniform over that interval.
Follow-up: How would you produce an integer from 0 through 9? Multiply by 10 and apply Math.floor(), yielding Math.floor(Math.random() * 10).
QHow do you scale Math.random() to a range?
Use Math.random() * (max - min) + min for a floating-point value in the interval [min, max). The lower bound may be returned, while the upper bound is excluded.
Follow-up: How would you include an integer upper bound? Use Math.floor(Math.random() * (max - min + 1)) + min for integer bounds.
QWhat are Math.random()'s security and seeding limitations?
No arguments are accepted. The function's pseudo-random output is not cryptographically secure, so security-sensitive randomness should come from Crypto.getRandomValues().
Follow-up: Why not seed Math.random() yourself? Its initial seed is selected by the implementation and cannot be chosen or reset by the user.
Related
Math.floor()Crypto.getRandomValues()Math