Simulate a map / set in Javascript

Question

I have a json object, say box = {}; to which I will add key values, such as box['somename'] = somevalue . There may be repetitions of somename , and I want to get the value of the last instance. All this is wonderful.

Now I need to operate as if it were an array. Basically, now that I have a set of unique keys, I need one basic box.length operation to see how many unique elements there are. Is there an elegant permanent way to do this without repeating all the properties of this object?

Answer 1

 var box = { length: 0, add: function(k, v) { if (typeof this[k] === 'undefined') this.length++; this[k] = v; } } 

Answer 2

Increase the counter every time you add a new item to the box .

 function Box() { var length = 0; var items = {}; this.add = function(k, v) { if (!(k in items)) length++; // don't count twice items[k] = v; } this.get = function(k) { return items[k]; } this.delete = function(k) { if (k in items) length--; delete items[k]; } this.__defineGetter__("length", function() { return length; }); } 

This version correctly handles adding and removing elements with any name and provides read-only access to the length property. Using:

 var box = new Box(); box.add("a", 1); box.add("a", 2); // overwrite box.add("b", "whatever"); box.add(null, 3); box.add(undefined, 3); box.add(undefined, 42); box.add("", 41); console.log(box.length); // 5 console.log(box.get(undefined)); // 42 console.log(box.get(null)); // 3 console.log(box.get("")); // 41 box.delete(undefined); box.delete(undefined); box.delete(undefined); box.delete(undefined); box.delete(undefined); box.delete(undefined); box.delete(22); // never was defined console.log(box.length); // 4 console.log(box.get(undefined)); // undefined box.add("length", "33") box.add("items", "jfhsdjkfh"); box.add("length", 77); console.log(box.length); // 6