const x = {};
    ‘constructor’ in x // true
    x[‘constructor’] === Object // true

Some properties cannot be deleted, and some cannot be modified:

    delete x.constructor;
    x.constructor === undefined // false
    x.__proto__ = undefined;
    x.__proto__ === undefined // false

    const s = new Set();

    const a = {}, b = {};

    s.add(a);

    s.has(a); // true

    s.has(b); // false


    const o = {};

    o[a] = true;

    o[a]; // true

    o[b]; // true, cause its cast to a string like "[object Object]"

It's O(1) for the latter (edit: I stand corrected, but probably close, as per spec).

Conversion to and from arrays even preserves order.

So I don't see these two use cases as "showing off", why should anybody do that?