Pick a random item from a list

pick.ts

/**
 * Picks a random item from a list.
 * 
 * For example:
 * 
 *    pick(['a', 'b', 'c'], [0.7, 0.2])
 *    // outputs 'a' with 70% chance
 *    // outputs 'b' with 20% chance
 *    // outputs 'c' with 10% chance (the rest)
 * 
 *    pick(['a', 'b', 'c'])
 *    // outputs 'a', 'b' or 'c' with the same probability
 */
const pick = <T>(items: T[], rates: number[] = []) => {
  if (!rates.length) {
    return items[Math.floor(Math.random() * items.length)]
  }

  const p = Math.random()
  let acc = 0
  for (const i in rates) {
    const rate = rates[i]
    if (p < acc + rate) {
      return items[i]
    }
    acc += rate
  }

  return items[items.length - 1]
}