How to create a Javascript Hash Table / Associate Array Prototype

Question

With a associative array such as:

var m = {};
m['a'] = {id:1, foo:'bar'};
m['b'] = {id:2, foo:'bar'};

Is it possible to create a prototype such as:

Array.prototype.hello = function() { console.log('hello'); }
m.hello();

This fails because m is an object, so I tired:

Object.prototype.hello = function() { console.log('hello'); }

and this is problematic too.

Is is possible to create a prototype which can operate on this data structure?

Update: I think I need some sleep :)

When I create and use the Object.prototype.hello = function() { console.log('hello'); } by itself it works fine.

When I add the prototype and include a 3rd party JS Framework, it makes the framework stop working.

user1106925 · Accepted Answer · 2015-05-30 16:55:11Z

2

Why not create your own object constructor so that you can extend its prototype without issues?

function O(o) {
    for (var p in o) {
        this[p] = o[p];
    }
}

O.prototype.hello = function() { console.log('hello') }

Then use the constructor with your object literals.

var m = new O({})
m['a'] = {id:1, foo:'bar'}
m['b'] = {id:2, foo:'bar'}

There are tricks that'll let you drop the new if you wish.

answered May 30, 2015 at 16:55

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Comments

T.J. Crowder · Accepted Answer · 2015-05-30 16:55:42Z

1

You can assign custom properties to any object, and that means you can do so on an object with a different immediate underlying prototype than Object.prototype. So you could do this, for instance:

function MyMap() {
}
MyMap.prototype.hello = function() {
    console.log('hello');
};
var m = new MyMap();
m['a'] = {id:1, foo:'bar'};
m['b'] = {id:2, foo:'bar'};
m.hello();

Note, though, that if you stored a hello entry:

m['hello'] = {id:3, foo:'bar'};

...it would hide the hello that your object gets from the prototype.

Also note that your m will have the properties not only from MyMap.prototype, but also from Object.prototype (like {} does), like toString and valueOf and hasOwnProperty. If you want to not have Objectproperties, you can do that, too:

function MyMap() {
}
MyMap.prototype = Object.create(null);
MyMap.prototype.hello = function() {
    console.log('hello');
};

Also note that constructor functions (MyMap, above) are only one way to create objects with an underlying prototype. You can just use Object.create directly:

var mapPrototype = {
    hello: function() {
        console.log('hello');
    }
};
var m = Object.create(mapPrototype);

answered May 30, 2015 at 16:55

T.J. Crowder

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2 Comments

Brian McGinity Over a year ago

I am going to try your last suggestion. Thank you for your help.

T.J. Crowder Over a year ago

@BrianMcGinity: No worries. Note that it has the same conflict issue that all the others do. :-) The only way around the conflicts problem would be to store the entries on a different object, like ES6's map conceptually does.

ryanve · Accepted Answer · 2015-05-30 16:56:49Z

1

You could use Object.create to create an array-like prototype for your structure.

var proto = Object.create(Array.prototype);
proto.hello = function() { console.log('hello'); }

Then use it like

var stack = Object.create(proto);
stack.hello();
stack.push('example');

answered May 30, 2015 at 16:56

ryanve

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Comments

Gerard van Helden · Accepted Answer · 2015-05-30 16:55:08Z

0

Assigning to Object.prototype should work just fine. When running in node:

> Object.prototype.foo = function() { console.log("foo!"); }
[Function]
> var m = {};
undefined
> m.foo();
foo!
undefined

Whether it is a good idea is a whole other discussion...

answered May 30, 2015 at 16:55

Gerard van Helden

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