What is happening in Crockford's object creation technique?

Question

There are only 3 lines of code, and yet I\'m having trouble fully grasping this:

Object.create = function (o) {
    function F() {}
    F.prototype = o;
             


        

Answer 1

> Clearly it won't be possible to have a constructor function using this technique.

The technique is already an object constructor since it returns new F(), but no property values can be set as for say new man('John','Smith'). However, if the Object.create code is modified, instantiation is possible. For example the sarah object below can be constructed and instantiated using Object.creator, and will inherit the getName method.

var girl = {
   name: '',
   traits: {},
   getName: function(){return this.name}
}

var sarah = Object.creator(girl, 'Sarah', {age:29,weight:90})

The sarah object will then consist of own properties { name:'Sarah', traits:{age:9,weight:49} }, and the prototype inherited sarah.getName() will produce 'Sarah'.

The following method relies on own properties enumerating with 'for(prop in o)' in creation order. Although not guaranteed by ECMA specs, this example (and a few more complex) worked for all major browsers (4) tested, providied hasOwnProperty() was used, otherwise not.

Object.creator = function(o) {
   var makeArgs = arguments 
   function F() {
      var prop, i=1, arg, val
      for(prop in o) {
         if(!o.hasOwnProperty(prop)) continue
         val = o[prop]
         arg = makeArgs[i++]
         if(typeof arg === 'undefined') break
         this[prop] = arg
      }
   }
   F.prototype = o
   return new F()
}

The official ECMA Object.create has an optional 2nd parameter, propertiesObject, that can instantiate property values, but it is an object rather than the usual list, and looks awkward to use. E.g. I believe:-

o2 = Object.create({}, { p: { value: 42, writable: true, enumerable: true, configurable: true } });

is equivalent to the much simpler old way:-

o2 = new function(p) { this.p=p }(42)

and

o2 = Object.creator({p:''}, 42)