Attempt at creating a Ruby-ish Hash using JavaScript's Proxy.

hash.js

'use strict';

const values = new Map();

const getValues = hash => {
  const v = values.get(hash) || new Map();
  values.set(hash, v);
  return v;
};

class Hash extends Map {
  constructor(...args) {
    super(...args);

    return new Proxy(this, {
      set(target, prop, value) {
        const desc = Reflect.getOwnPropertyDescriptor(target, prop);
        if (desc && typeof desc.value === 'function') {
          return target[prop](value);
        }
        if (desc && typeof desc.set === 'function') {
          target[prop] = value;
          return value;
        }

        target.set(prop, value);
        return value;
      },
      get(target, prop, receiver) {
        if (!(prop in target)) {
          return target.get(prop);
        }
        if (typeof target[prop] === 'function') {
          return target[prop].bind(target);
        }
        return target[prop];
      }
    });
  }

  set(key, value) {
    const values = getValues(this);
    values.set(value, key);
    return super.set(key, value);
  }

  delete(key) {
    const values = getValues(this);
    const value = this.get(key);
    values.delete(value);
    return super.delete(key);
  }

  key(value) {
    const values = getValues(this);
    return values.get(value);
  }
}

const hash = new Hash([['foo', 'bar'], ['baz', 'qux']]);

hash.baz = 'whatever';
console.log(hash.key('bar'));
console.log(hash.has('baz'));