Ordered hash in JavaScript

Question

JavaScript objects have no order stored for properties (according to the spec). Firefox seems to preserve the order of definition of properties when using a for...in loop. Is this behaviour something that I can rely on? If not is there a piece of JavaScript code somewhere that implements an ordered hash type?

Answer 1

JavaScript in 2016, specifically EcmaScript 6, supports the Map built-in class.

A Map object iterates its elements in insertion order — a for...of loop returns an array of [key, value] for each iteration.

That's what you need. (I wonder why that is the first info in the description of this data structure, though.)

For example,

m = new Map()  m.set(3,'three') m.set(1,'one') m.set(2,'two')  m // Map { 3 => 'three', 1 => 'one', 2 => 'two' }  [...m.keys()] // [ 3, 1, 2 ] 

or the example from the docs:

var myMap = new Map(); myMap.set(0, 'zero'); myMap.set(1, 'one');  myMap // Map { 0 => 'zero', 1 => 'one' }  for (var [key, value] of myMap) {   console.log(key + " = " + value); }  for (var key of myMap.keys()) {   console.log(key); }  for (var value of myMap.values()) {   console.log(value); }  for (var [key, value] of myMap.entries()) {   console.log(key + " = " + value); }