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(). That means:

var a = {foo: 1};
var b = {bar: 2};
var o = {};
o[a] = 100;
o[b];              // 100
JSON.stringify(o); // '{"[object Object]":100}'

That is, since the toString() of any plain Object is [object Object], they all address the same value.

I’d like to create a hashmap where Objects with the same properties and values address the same value, but objects with different properties or values address different values. That is:

var a = {foo: 1};
var b = {bar: 2, baz: 3};
var c = {baz: 3, bar: 2};
var hash = new Hash();
hash.set(a, 100);
hash.get(b);      // undefined
hash.set(b, 200);
hash.get(b);      // 200
hash.get(c);      // 200

My first instinct was to use JSON.stringify() to turn objects into strings, but:

var hash = {};
var b = {bar: 2, baz: 3};
var c = {baz: 3, bar: 2};
hash[JSON.stringify(b)] = 100
hash[JSON.stringify(b)] // 100
hash[JSON.stringify(c)] // undefined
JSON.stringify(b)       // '{"bar":2,"baz":3}'
JSON.stringify(c)       // '{"baz":3,"bar":2}'

That is, JSON serialization is order-dependent.

Is there a good library or technique to implement a hashmap like this?

Update:

Equivalently, is there a good hashing function such that:

hash({foo: 1, bar: 2}) == hash({bar: 2, foo: 1})

Answer 1

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 = {};
}