How to customize object equality for JavaScript Set

Question

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:

Answer 1

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.