How to customize object equality for JavaScript Set

New ES 6 (Harmony) introduces new Set object. Identity algorithm used by Set is similar to === operator and so not much suitable for comparing objects:

var set = new Set();
set.add({a:1});
set.add({a:1});
console.log([...set.values()]); // Array [ Object, Object ]

How to customize equality for Set objects in order to do deep object comparison? Is there anything like Java equals(Object)?

The ES6 Set object does not have any compare methods or custom compare extensibility.

The .has(), .add() and .delete() methods work only off it being the same actual object or same value for a primitive and don't have a means to plug into or replace just that logic.

You could presumably derive your own object from a Set and replace .has(), .add() and .delete() methods with something that did a deep object comparison first to find if the item is already in the Set, but the performance would likely not be good since the underlying Set object would not be helping at all. You'd probably have to just do a brute force iteration through all existing objects to find a match using your own custom compare before calling the original .add().

Here's some info from this article and discussion of ES6 features:

5.2 Why can’t I configure how maps and sets compare keys and values?

Question: It would be nice if there were a way to configure what map keys and what set elements are considered equal. Why isn’t there?

Answer: That feature has been postponed, as it is difficult to implement properly and efficiently. One option is to hand callbacks to collections that specify equality.

Another option, available in Java, is to specify equality via a method that object implement (equals() in Java). However, this approach is problematic for mutable objects: In general, if an object changes, its “location” inside a collection has to change, as well. But that’s not what happens in Java. JavaScript will probably go the safer route of only enabling comparison by value for special immutable objects (so-called value objects). Comparison by value means that two values are considered equal if their contents are equal. Primitive values are compared by value in JavaScript.

As mentioned in jfriend00's answer customization of equality relation is probably not possible.

Following code presents an outline of computationally efficient (but memory expensive) workaround:

class GeneralSet {

    constructor() {
        this.map = new Map();
        this[Symbol.iterator] = this.values;
    }

    add(item) {
        this.map.set(item.toIdString(), item);
    }

    values() {
        return this.map.values();
    }

    delete(item) {
        return this.map.delete(item.toIdString());
    }

    // ...
}

Each inserted element has to implement toIdString() method that returns string. Two objects are considered equal if and only if their toIdString methods returns same value.

As the top answer mentions, customizing equality is problematic for mutable objects. The good news is (and I'm surprised no one has mentioned this yet) there's a very popular library called immutable-js that provides a rich set of immutable types which provide the deep value equality semantics you're looking for.

Here's your example using immutable-js:

const { Map, Set } = require('immutable');
var set = new Set();
set = set.add(Map({a:1}));
set = set.add(Map({a:1}));
console.log([...set.values()]); // [Map {"a" => 1}]