Question

Is there a way to give objects in js custom hashes, just as overriding

__hash__()

in python let’s someone define how a given object is hashed into a dictionary.

My underlying question is: what hash function is used to put js objects into associative arrays, and can I over-ride it?

Answer 1

You mean using objects as keys, how do you make sure you access that key again?

The magic method is toString(). Turns out all objects in JS use string keys, and the toString() method is called if it’s not a string.

http://jsfiddle.net/udsdT/1/

var objA = {
    data: 'yay',
    toString: function() {
        return 'value_as_key';
    }
};

var objB = {
    data: 'some other totally data thing',
    toString: function() {
        return 'value_as_key';
    }
}


var foo = {};
foo[objA] = 'abc';
foo[objB] = 'def';
foo['value_as_key'] = 'qwe';
foo.value_as_key = 'omg';

foo[objA];           // 'omg'
foo[objB];           // 'omg'
foo['value_as_key']; // 'omg'
foo.value_as_key;    // 'omg'

​​​​​​Usually though, you really don’t want to use whole objects as keys. Especially if you dont create your own toString() method, since the default toString() method on basic objects isn’t exactly very awesome…

({a:123}).toString() // [object Object]
({b:456}).toString() // [object Object]

var foo = {};
foo[{a:123}] = 'wtf';
foo[{asdf:9876}];       // 'wtf'
foo['[object Object]']; // 'wtf