protocols.js

class ProtocolMap {
  constructor() {
    this.impls = new Map();
    this.cache = new Map();
  }

  get(object) {
    let proto = Object.getPrototypeOf(object);
    let impl = this.cache.get(proto);

    if (impl) {
      return impl;
    }

    while (proto !== null) {
      impl = this.impls.get(proto);

      if (impl) {
        this.cache.set(Object.getPrototypeOf(object), impl);
        return impl;
      }

      proto = Object.getPrototypeOf(proto);
    }

    return undefined;
  }

  set(prototype, impl) {
    this.impls.set(prototype, impl);
    this.cache = new Map([[prototype, impl]]);
  }

  call(object, ...args) {
    return this.get(object)(object, ...args);
  }
}

const protocols = new Map();
protocols.set(Symbol.for('IGet'), new ProtocolMap());
protocols.set(Symbol.for('ISet'), new ProtocolMap());
const IGet = protocols.get(Symbol.for('IGet'));
const ISet = protocols.get(Symbol.for('ISet'));
IGet.set(Map.prototype, (m, k) => m.get(k));
IGet.set(Object.prototype, (m, k) => m[k]);
ISet.set(Map.prototype, (m, k, v) => m.set(k, v));
ISet.set(Object.prototype, (m, k, v) => m[k] = v);

class FooMap extends Map {}

const a = new FooMap();
const b = {};

ISet.call(a, "foo", "bar")
ISet.call(b, "baz", "qux")

console.log(IGet.call(a, "foo"));
console.log(IGet.call(b, "baz"));

// ** OUTPUT **
// bar
// qux