weighted-choices.js

/** Get a random integer 0 <= i < max */
function getRandomInteger(max) {
  return Math.floor(Math.random() * max);
}
function getWeightedChoice(choices, weights) {
  const sumOfWeights = weights.reduce((acc, x) => acc + x, 0);
  let randomInt = getRandomInteger(sumOfWeights) + 1;
  for (const [index, weight] of weights.entries()) {
    randomInt = randomInt - weight;
    if (randomInt <= 0) {
      return choices[index];
    }
  }
  throw new Error('This shouldn’t happen');
}
console.log(getWeightedChoice(['a', 'b'], [50, 50]));

Different approach (not necessarily better):

function getWeightedChoice(choices, weights) {
    return choices
        .map((c,i) => [c, Math.floor(Math.random() * weights[i])])
        .sort((a,b)=>b[1]-a[1])
        [0][0]
};