returns random outcomes based on a configuration of their weights

weightedRandom.mjs

// array of [outcome, weight]
export function weightedRandom(arr) {
    const outcomeCuts = [];
    let accum = 0;
    for (const [out, weight] of arr) {
        accum += weight;
        outcomeCuts.push([out, accum]);
    }
    const r = accum * Math.random();
    let best;
    outcomeCuts.reverse();
    for (const [out, cutAt] of outcomeCuts) {
        if (r <= cutAt) best = out;
        else break;
    }
    return best;
}

weightedRandom.test.mjs

import test from 'node:test';
import { ok } from 'node:assert/strict';

import { weightedRandom } from './weightedRandom.mjs';

test('weightedRandom', (_t) => {
    const config = [
        ['a', 60], // 60% or 0.6
        ['b', 30], // 30% or 0.3
        ['c', 10], // 10% or 0.1
    ];
    const numRuns = 10000;
    const histo = { a: 0, b: 0, c: 0 };
    for (let i = 0; i < numRuns; ++i) {
        const outcome = weightedRandom(config);
        ++histo[outcome];
    }
    for (const k of Object.keys(histo)) histo[k] /= numRuns;

    const e = 0.01;

    ok(histo.a > 0.6 - e && histo.a < 0.6 + e);
    ok(histo.b > 0.3 - e && histo.b < 0.3 + e);
    ok(histo.c > 0.1 - e && histo.c < 0.1 + e);
});