comparing ECMA6 sets for equality

Question

How do you compare two javascript sets? I tried using == and === but both return false.

a = new Set([1,2,3]);
b = new Set([1,3,2]);
a == b; //=> false
a === b; //=> false

These two sets are equivalent, because by definition, sets do not have order (at least not usually). I've looked at the documentation for Set on MDN and found nothing useful. Anyone know how to do this?

Answer 1

Try this:

var a = new Set([1,2,3]);
var b = new Set([1,3,2]);

alert(eqSet(a, b)); // true

function eqSet(as, bs) {
    if (as.size !== bs.size) return false;
    for (var a of as) if (!bs.has(a)) return false;
    return true;
}

A more functional approach would be:

var a = new Set([1,2,3]);
var b = new Set([1,3,2]);

alert(eqSet(a, b)); // true

function eqSet(as, bs) {
    return as.size === bs.size && all(isIn(bs), as);
}

function all(pred, as) {
    for (var a of as) if (!pred(a)) return false;
    return true;
}

function isIn(as) {
    return function (a) {
        return as.has(a);
    };
}

The all function works for all iterable objects (e.g. Set and Map).

If Array.from was more widely supported then we could have implemented the all function as:

function all(pred, as) {
    return Array.from(as).every(pred);
}

Hope that helps.