Simulating map/set in Javascript

Question

I have a json object, say box = {}; to which I will keep adding key-values like box['somename'] = somevalue. There may be repetitions of somename and I want the last instance's value to win. All this is fine.

Now I need to operate on it, as if it were an array. Basically, now that I have a set of unique keys, I want one main operation box.length to see how many unique elements there are. Is there an elegant constant time way of doing it without iterating through all properties of this object?

possible duplicate of [How to efficiently count the number of keys/properties of an object in JavaScript?](http://stackoverflow.com/questions/126100/how-to-efficiently-count-the-number-of-keys-properties-of-an-object-in-javascript) — Lightness Races in Orbit, Mar 02 '11 at 19:27
yes, looks like that, but the answers here are much better, don't you think :) — Sanjeev Satheesh, Mar 02 '11 at 19:41

score 9 · Accepted Answer · answered Mar 02 '11 at 19:31

9

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

answered Mar 02 '11 at 19:31

Eric Mickelsen

9,511
2
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1

You should also check whether `k != "length"` before you make the assignment. – Jan Mar 02 '11 at 19:33
@Jan: True, the add method should probably raise an error if k is "length" or any other special property of box. – Eric Mickelsen Mar 02 '11 at 19:35
2

What if you reomve a key/value pair (`delete box[somekey]`)? – KooiInc Mar 02 '11 at 19:42
@Kooilnc: `delete: function(k) { if (typeof this[k] !== 'undefined') this.length--; delete this[k]; }` – Eric Mickelsen Mar 02 '11 at 19:44
@Eric Mickelsen: and what if you don't use `add` (`box.somenewkey = 'directset'`)? – KooiInc Mar 02 '11 at 19:58
1

@Kooilnc: In that case, you're SOL no matter what. There is no way in Javascript to prevent such a thing. The presumption here is that you will use the provided functions consistently. – Eric Mickelsen Mar 02 '11 at 20:12

Wayne · Answer 2 · 2011-03-03T01:39:18.857

Increment a counter every time you add a new element to 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. Usage:

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

@Eric - Fixed, but I like your solution better anyway. Much more JavaScripty. — Wayne, Mar 02 '11 at 19:37
Probably nit-picking, but yours is a more robust approach, since the value itself can be `undefined`. Calling `box.addItem("a", undefined)` multiple times will not increment the `length`. +1 — Anurag, Mar 02 '11 at 20:21

Simulating map/set in Javascript

2 Answers2