findIndex, missing findIndexes, prototype, polyfill

find_indexes.js

const findIndexes = (arr, fn) => {
  const res = [];
  arr.map((el, i, arr) => {
    if (fn(el, i, arr)) res.push(i);
  });
  return res;
};

// set prototype
Array.prototype.findIndexes = function(fn) {
  return findIndexes(this, fn);
};