membranes and getPrototypeOf

membrane_getPrototypeOf.js

load('reflect.js'); // https://github.com/tvcutsem/harmony-reflect/blob/master/reflect.js

function makeMembrane(initTarget) {
  var cache = new WeakMap();
  var revoked = false;
  
  function wrap(obj) {
    if (Object(obj) !== obj) return obj; // primitives are passed through
    var wrapper = cache.get(obj);
    if (wrapper) return wrapper;
    wrapper = Proxy(obj, Proxy(obj, { // "double lifting"
      get: function(target, trapName) {
        if (revoked) throw new TypeError("membrane revoked");
        return function(target /*, ...args*/) { // generic trap
          var args = Array.prototype.slice.call(arguments, 1).map(wrap);
          return wrap(Reflect[trapName].apply(undefined, [target].concat(args)));
        }
      }
    }));
    cache.set(obj, wrapper);
    return wrapper;
  }
  
  function revoke() { revoked = true; }
  
  return {revoke: revoke, target: wrap(initTarget)};
}

// David's example, updated to work with above membrane code:
function C(){};
var membrane = makeMembrane(C);
var wrappedC = membrane.target;

// Thanks to the wrapping get trap, we have:
print(wrappedC.prototype !== C.prototype);
   
// Now, consider:
var wrappedCInstance = new wrappedC(); // created a cInstance as target
   
// Because of the getPrototypeOf invariant, we have:
print(Object.getPrototypeOf(wrappedCInstance) === Object.getPrototypeOf(cInstance));
// the above line will throw: TypeError: prototype value does not match

print("done!");