Best way to store a key=>value array in JavaScript?

That’s just what a JavaScript object is:

var myArray = {id1: 100, id2: 200, "tag with spaces": 300};
myArray.id3 = 400;
myArray["id4"] = 500;

You can loop through it using for..in loop:

for (var key in myArray) {
  console.log("key " + key + " has value " + myArray[key]);
}

See also: Working with objects (MDN).

In ECMAScript6 there is also Map (see the browser compatibility table there):

• An Object has a prototype, so there are default keys in the map. This could be bypassed by using map = Object.create(null) since ES5, but was seldomly done.
• The keys of an Object are Strings and Symbols, where they can be any value for a Map.
• You can get the size of a Map easily while you have to manually keep track of size for an Object.