Weighted random choice in JavaScript, takes an array of non-negative weights

weighted-choice.js

function weightedChoice(weights) {
  const weightSum = weights.reduce((sum, w) => sum + w)
  let choice = Math.floor(Math.random() * weightSum) + 1
  let idx = weights.length - 1
  while ((choice -= weights[idx]) > 0) {
    idx -= 1
  }
  return idx
}