Thinking about what the new operator really looks like, if you treat Object.create() as the more fundamental operation

new.js

function new_(constructor /* arguments to constructor */) {
  var args = Array.prototype.slice.call(arguments, 1);
  var obj = Object.create(constructor.prototype);
  var result = constructor.apply(obj, args);
  if (result === undefined) {
    result = obj;
  }
  return result;
}

// Equivalent:
// new Constructor(a, b)
// new_(Constructor, a, b)