Reference indexed collection in Javascript

Question

Reference indexed collection in Javascript

I search a way to get in Javascript an object which index by reference.

something like:

var myRefMap = new RefMap();

var a = [{}, [], function () {}];

myRefMap.set(a[0], true);
myRefMap.set(a[1], { hello: 'world' });
myRefMap.set(a[2], 42);

myRefMap.has(a[0]); // true
myRefMap.has(a[1]); // true
myRefMap.has(a[2]); // true

myRefMap.has({});             // false
myRefMap.has([]);             // false
myRefMap.has(function () {}); // false

I need this object in order to optimize and avoid an actual heavy array search, during tree browsing within circulars references.

Thanks,

javascript
node.js
firefox
google-chrome
v8

Answer 1

If your code is saying that you want that last condition to evaluate to true, then just using an object and the 'in' directive should work fine.

var mystuff = {};
var a = [{}, [], function () {}];

mystuff[a[0]] = true;
mystuff[a[1]] = {hello:'world'};
mystuff[a[2]] = 42;

(a[0] in mystuff); // true
({} in mystuff);   // true
(a[1] in mystuff); // true
(a[2] in mystuff); // true

If however you wanted ({} in mystuff) to evaluate to false, that's a lot different.

Answer 2

One basic way to do this, provided all the stored values are objects would be something like:

(function () {
    var instCount = 0;
    self.RefMap = function () {
        instCount ++;
        var propName = '__RefMap' + instCount + '__';
        this.has = function (o) {
            return propName in o;
        };
        this.set = function (o) {
            o[propName] = true;
        }
        this.remove = function (o) {
            delete o[propName];
        }
    }
}());

//then use it like
var myRefMap = new RefMap();

Edit: Arrays and functions are objects too. And you can use Object.defineProperty to hide the "marking" property, depending on the browsers you target.