weightedSample

Returns a random element from an array, using the provided weights as the probabilities for each element.

Use Array.prototype.reduce() to create an array of partial sums for each value in weights. Use Math.random() to generate a random number and Array.prototype.findIndex() to find the correct index based on the array previously produced. Finally, return the element of arr with the produced index.

const weightedSample = (arr, weights) => {
  let roll = Math.random();
  return arr[
    weights
      .reduce(
        (acc, w, i) => (i === 0 ? [w] : [...acc, acc[acc.length - 1] + w]),
        []
      )
      .findIndex((v, i, s) => roll >= (i === 0 ? 0 : s[i - 1]) && roll < v)
  ];
};

weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9