What modernizer scripts exist for the new ECMAScript 5 functions?

Question

ECMAScript 5 has quite a few nice additions. John Resig has a good overview here. Here is a good ECMAScript 5 compatibility table.

A lot of this stuff can be \"fake

Answer 1

Crockford recommends this kind of Object.create shim:

if (typeof Object.create != "function") {
  Object.create = function (o) {
    function F(){}
    F.prototype = o;
    return new F;
  };
}

But please don't do this.

The problem with this approach is that ES5 Object.create has a signature of 2 arguments: first — an object to inherit from, and second (optional) — an object representing properties (or rather, descriptors) to add to newly created object.

Object.create(O[, Properties]); // see 15.2.3.5, ECMA-262 5th ed.

What we have is an inconsistent implementation with 2 different behaviors. In environments with native Object.create, method knows how to handle second argument; in environments without native Object.create, it doesn't.

What are the practical implications?

Well, if there's some code (say, a third party script) that wants to use Object.create, it's rather reasonable for that code to do this:

if (Object.create) {
  var child = Object.create(parent, properties);
}

— essentially assuming that if Object.create exists, it must conform to specs — accept second argument and add corresponding properties to an object.

But, with the above-mentioned shim, second argument is simply ignored. There's not even an indication of something going wrong differently. A silent failure, so to speak — something that's rather painful to detect and fix.

Can we do better?

Well, it's actually impossible to create a fully-conforming Object.create shim using only (standard) ES3 facilities. The best solution is to create a custom wrapper method.

There are, however, few alternative (less than optimal) things you can try:

1) Notify user about inability to work with second argument

if (!Object.create) {
  Object.create = function (o) {
    if (arguments.length > 1) { 
      throw Error('second argument is not supported'); 
    }
    // ... proceed ...
  };
}

2) Try to handle second argument:

if (!Object.create) {
  Object.create = function (parent, properties) {
    function F(){}
    F.prototype = parent;
    var obj = new F;
    if (properties) {
      // ... augment obj ...
    }
    return obj;
  }; 
}

Note that "properties" is an object representing property descriptors, not just property names/values, and is something that's not very trivial to support (some things are not even possible, such as controlling enumerability of a property):

Object.create(parent, {
  foo: {
    value: 'bar',
    writable: true
  },
  baz: {
    get: function(){ return 'baz getter'; },
    set: function(value){ return 'baz setter'; },
    enumerable: true
  }
});

The other inconsistency in the original shim is that it doesn't take care of parent object being null.

var foo = Object.create(null);

This creates an object whose [[Prototype]] is null; in other words, object that doesn't inherit from anything, not even Object.prototype (which all native objects in ECMAScript inherit from).

foo.toString; // undefined
foo.constructor; // undefined
// etc.

This is, by the way, useful to create "proper" hash tables in ECMAScript.

It's possible to emulate this behavior, but only using non-standard extensions, such as "magical" __proto__ property (so implementation would be not very portable or robust). Solution to this problem is similar: either emulate ES5 implementation fully, or notify about inconsistency/failure.