JavaScript Random Number Generator

Math.random() returns a float in [0, 1). Multiply by the number of possible values, floor it, and add the minimum.

function randomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min; // min..max inclusive
}

randomInt(1, 10);

Scale Math.random() to any range of decimals.

Math.random();                       // 0 <= x < 1
const x = Math.random() * (5 - 1.5) + 1.5; // 1.5 <= x < 5

Pick an index at random, or shuffle with Fisher-Yates - sorting with () => Math.random() - 0.5 gives a biased order.

const items = ["red", "green", "blue", "yellow"];
const pick = items[Math.floor(Math.random() * items.length)];

// Fisher-Yates shuffle (in place)
for (let i = items.length - 1; i > 0; i--) {
  const j = Math.floor(Math.random() * (i + 1));
  [items[i], items[j]] = [items[j], items[i]];
}

Math.random() is not cryptographically secure. For tokens, IDs and draws use the Web Crypto API (browsers and Node 19+), or crypto.randomInt in Node.

const bytes = crypto.getRandomValues(new Uint8Array(16)); // 16 random bytes
const id = crypto.randomUUID();                            // random UUID v4

// Node.js
const { randomInt } = require("node:crypto");
randomInt(1, 11); // 1..10 (upper bound exclusive)

Math.round(Math.random() * 10) makes 0 and 10 half as likely as the other values; Math.floor(Math.random() * 10) never returns 10. Use the floor formula with max - min + 1.