weakest-of-maps.js

if (typeof WeakMap !== 'function') {
	let c = 0;
	WeakMap = function() {
		let id = typeof Symbol === 'function' ? Symbol() : `__weak$${++c}`;
		this.set = (key, val) => { key[id] = val };
		this.get = key => key[id];
	};
}

function createMap() {
  if (typeof Map=='function') return new Map();
  const keys=[], values=[];
  return {
    get(key) {
      const i = keys.indexOf(key);
      if (i!==-1) return values[i];
    },
    set(key, value) {
      const i = keys.indexOf(key);
      if (i!==-1) values[i] = value;
      else values[keys.push(key)] = value
    }
  };
}