filterMap 1.0.0

/**
 * Calls a function on each item of an array and returns the results, leaving
 * out any item the function skips. It's like `map` and `filter` in one pass.
 * To skip an item, return the `skip` value that the function is given as its
 * fourth argument. Every other result is kept, including `undefined`.
 * @template T, U
 * @param {ArrayLike.<T>} arr
 *    The array, or array-like object (such as a string or a NodeList), to go
 *    through.
 * @param {function(T, number, ArrayLike.<T>, Object): U} fn
 *    Called with each item, its index, `arr` and `skip`. Returns the value to
 *    keep, or `skip` to leave the item out.
 * @returns {Array.<U>}
 *    The results of `fn`, in order, without the items it skipped.
 *
 * @example <caption>Map and filter in one pass</caption>
 * filterMap([1, 2, 3, 4], (n, i, arr, skip) => n % 2 ? skip : n * 10);
 * // → [20, 40]
 *
 * @example <caption>Keeping only the numbers</caption>
 * filterMap(['1', 'two', '3'], (s, i, arr, skip) => {
 *   const n = Number(s);
 *   return Number.isNaN(n) ? skip : n;
 * });
 * // → [1, 3]
 *
 * @example <caption>undefined is kept like any other value</caption>
 * filterMap([1, undefined, 2], (x, i, arr, skip) => x === 2 ? skip : x);
 * // → [1, undefined]
 */
function filterMap(arr, fn) {
  // A new object each call, so nothing a caller returns can match it by accident
  var skip = {};
  var ret = [];
  for (var i = 0, l = arr.length; i < l; i++) {
    var result = fn(arr[i], i, arr, skip);
    if (result !== skip) {
      ret.push(result);
    }
  }
  return ret;
}