bracket-map.js

'use strict';

var bracketMapKey = Symbol('bracketMapKey');

Object.prototype[Symbol.toPrimitive] = function () {
	return this[bracketMapKey] || (this[bracketMapKey] = Symbol());
};

var map = {};
var key = ['some object'];

map[key] = 5;

console.log(key in map); // true
console.log(['some object'] in map); // false