weightedSelector.js

function selector(weights) {
  let maxRandom = [];
  let curr = 0;
  for (let i = 0; i < weights.length; ++i) {
    let currentWeight = weights[i];
    curr += currentWeight;
    maxRandom.push(curr);
  }
  let randomNum = Math.random();

  for (let i = 0; i < maxRandom.length; ++i) {
    if (randomNum  <= maxRandom[i]) {
      return i;
    }
  }
}

let weights = [0.3, 0.2, 0.5];
let counts = [0, 0, 0];

const NUM_TRIALS = 5000;
for (let i = 0; i < 5000; ++i) {
  ++counts[selector(weights)];
}

for (let i = 0; i < counts.length; ++i) {
  console.log(`Index ${i}'s real probability is ${counts[i]/NUM_TRIALS}`);
  console.log(`Index ${i}'s expected probability is ${weights[i]}`);
}