Pick a weighted value from an object in JS

pickWeightedValue.js

const rules = {
  '90%': 0.9,
  '10%': 0.1
}

function pickWeightedValue(rules) {
  const productions = Object.keys(rules)
  const weights = Object.values(rules)

  let maxIndex = 0
  let maxWeight = 0
  let maxRoll = 0

  for (let i=0; i<weights.length; i++) {
    let currentWeight = weights[i]
    let currentRoll =  Math.pow(Math.random(), 1.0 / currentWeight)

    if (currentRoll > maxRoll) {
      maxRoll = currentRoll
      maxWeight = currentWeight
      maxIndex = i
    }
  }

  return productions[maxIndex]
}

const buckets = { '90%': 0, '10%': 0 }

for (z=0; z<1000; z++) {
  let result = pickWeightedValue(rules)
  buckets[result] = buckets[result] + 1
}

console.log(buckets)