JavaScript Math.random: Map a Fraction to an Integer Range

The task

Choose an integer from four through nine, inclusive. Inject the fractional source so boundary tests remain repeatable, then check a real draw only for its supported range.

Runnable Node 22 example

const assert = require('node:assert/strict');

function pick(low, high, random = Math.random) {
  assert(Number.isInteger(low) && Number.isInteger(high) && high >= low);
  const fraction = random();
  assert(fraction >= 0 && fraction < 1);
  return low + Math.floor(fraction * (high - low + 1));
}
assert.equal(pick(4, 9, () => 0), 4);
assert.equal(pick(4, 9, () => 0.5), 7);
assert.equal(pick(4, 9, () => 0.999999), 9);
const actual = Math.random();
assert(actual >= 0 && actual < 1);
console.log([0, 0.5, 0.999999].map(value => pick(4, 9, () => value)).join(','));

Expected output and explanation

There are six candidate integers, so multiplying by six creates six intervals before floor selects a position. Adding the lower bound shifts that position into the requested range. Testing fractions near both ends catches missing +1 and upper-bound mistakes.

The printed values are deterministic because those calls use injected fractions. The separate real draw is checked without printing a guessed value. These assertions establish the mapping for the selected boundaries, not statistical fairness of a large random experiment.

Pitfall to check

This helper assumes a modest interval within safe arithmetic limits. Do not use Math.random for secrets, security tokens or cryptographic choices; use the appropriate cryptographic API and its documented contract.