reverseIterator.js

// Motivation: we need a reverse iterator for list objects. // For Arrays, only reduceRight or decrementing loops // Currently impossible in Map/Set without a full loop or storing the list into an Array

// Goal: a iterator that allows looping through the elements of a list from the last to the first element. Similar to .values() // Goal: it should not change the original array // Goal: it should allow interrupting the iteration.

// Question: Should add equivalents to keys and entries? // Question: Names for the methods?

Symbol.reverse = Symbol('reverseIterator');

function *reverse() { if (!this || typeof this !== 'object') { throw new TypeError('this is not an Object'); } const arr = this; let { length } = arr;

while (length) { length = --length; if (!Object.prototype.hasOwnProperty.call(arr, length)) { continue; } yield arr[length]; // Dynamically jump to the last item in the array if it's shorter if (arr.length < length) { length = arr.length; // .next will jump to the latest existing value } } }

Array.prototype[Symbol.reverse] = reverse;

var array1 = [1, 2, 3, 4, 5]; console.log([...array1Symbol.reverse]); // [5, 4, 3, 2, 1];

var obj = { length: 7, 6: 'a', get 5() { this.length = 1; return 'b'; }, 4: 'z', 1: 'z', 0: 'c' };

console.log([...Array.prototype[Symbol.reverse].call(obj)]); // [ 'a', 'b', 'c']

var array2 = [1,,2,,,3]; console.log([...array2Symbol.reverse]); // [ 3, 2, 1 ]

/* ------------------- */

// As methods:

Array.prototype.reverseIterator = Array.prototype[Symbol.reverse]; // Needs bikesheding on the name!!!

// Map and Set not drafted here Map.prototype.reverse = Map.prototype[Symbol.reverse]; Set.prototype.reverse = Set.prototype[Symbol.reverse];