Is there a hashmap library for JavaScript?

Question

In JavaScript, all Objects act a bit like hashmaps. However, the keys to these hashmaps must be strings. If they\'re not, they\'re converted with toString(). Tha

Answer 1

This is an old question, but ES6 has a feature that may be relevant: Map

Maps can have arbitrary objects as keys, and arbitrary values as values. The main distinction is that the objects used as keys stay unique, even if objects are identical.

Here's an example:

var map = new WeakMap();

var o1 = {x: 1};
var o2 = {x: 1};

map.set(o1, 'first object');
map.set(o2, 'second object');

// The keys are unique despite the objects being identical.

map.get(o1); // 'first object'
map.get(o2); // 'second object'
map.get({x: 1}); // undefined

JSON.stringifying the objects wouldn't be able to distinguish between o1 and o2.

MDN has more info. There's also a WeakMap which doesn't maintain references to the objects used as keys, so they may be garbage collected.

The Traceur compiler doesn't yet have official polyfills for Map and Weakmap, but there is an open pull request with polyfills for both. The code for those polyfills (in case anyone wants to add them individually) are here: Map and WeakMap. Assuming those polyfills work well, you can start using Map or WeakMap today. :)

Answer 2

Prototype has Hash quite nicely covered http://prototypejs.org/api/hash

Answer 3

This thread actually led me to find a bug in my current project. However, this is fixed now and my HashMap implementation in my project (https://github.com/Airblader/jsava) will do exactly what you described. Unlike jshashtable, there are no buckets.

Answer 4

You could implement your own class with a toString that ensures an ordering of keys.

Answer 5

jOrder could be modified to treat objects with the same properties (but in different order) as identical when performing an index lookup.

If you have the object {bar: 2, baz: 3, val: 200} in your list, and you've previously put a fitting index on a jOrder table like this:

var table = jOrder(data)
    .index('signature', ['bar', 'baz'], {grouped: true});

Then right now table.where([{bar: 2, baz: 3}])[0].val will return 200, but table.where([{baz: 3, bar: 2}])[0].val won't. It wouldn't take much effort though to make it not depend on the order of properties. Let me know if you're interested and I'll push the fix to GitHub as soon as I can.

Answer 6

Here's a quick proof-of-concept...

I've hardly tested it at all, and I'm certain that there will be corner-cases that it can't deal with.

Performance will be hideously inefficient because the __createHash function needs to recurse through the members of any objects and then sort them, in order to generate a "hash" that meets your requirements.

HashMap = function() {
    this.get = function(key) {
        var hash = this.__createHash(key);
        return this.__map[hash];
    };

    this.set = function(key, value) {
        var hash = this.__createHash(key);
        this.__map[hash] = value;
    };

    this.__createHash = function(key) {
        switch (typeof key) {
            case 'function':
                return 'function';

            case 'undefined':
                return 'undefined';

            case 'string':
                return '"' + key.replace('"', '""') + '"';

            case 'object':
                if (!key) {
                    return 'null';
                }

                switch (Object.prototype.toString.apply(key)) {
                    case '[object Array]':
                        var elements = [];
                        for (var i = 0; i < key.length; i++) {
                            elements.push(this.__createHash(key[i]));
                        }
                        return '[' + elements.join(',') + ']';

                    case '[object Date]':
                        return '#' + key.getUTCFullYear().toString()
                                   + (key.getUTCMonth() + 1).toString()
                                   + key.getUTCDate().toString()
                                   + key.getUTCHours().toString()
                                   + key.getUTCMinutes().toString()
                                   + key.getUTCSeconds().toString() + '#';

                    default:
                        var members = [];
                        for (var m in key) {
                            members.push(m + '=' + this.__createHash(key[m]));
                        }
                        members.sort();
                        return '{' + members.join(',') + '}';
                }

            default:
                return key.toString();
        }
    };

    this.__map = {};
}