Generate a weighted random number in given range

Return an integer in [min, max) skewed toward min.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function weightedRandom(min, max) { return Math.floor(Math.random() ** 2 * (max - min)) + min; }

Usage

const draws = Array.from({ length: 10000 }, () => weightedRandom(0, 10));
draws.every(n => n >= 0 && n < 10); // → true
draws.filter(n => n < 5).length > draws.filter(n => n >= 5).length; // → true

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Squaring a uniform number skews results toward min: about 71% of draws land in the lower half, which the example checks. max is excluded.
• For explicit weights per outcome, pick from a table of cumulative weights instead.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Generate a random boolean — Return true or false with equal probability.
• Generate a random floating point number in given range — Scale Math.random into [min, max).
• Generate a random hex color — Return a #rrggbb color from 24 random bits.