My JavaScript book is out! Don't miss the opportunity to upgrade your beginner or average dev skills.

Friday, August 20, 2010

Object.defineProperty ... but Strict!

In my precedent post entitled A Pascal record via JavaScript I have showed a basic function able to emulate type hints behavior via JavaScript.
Even if that was a proof of concept, I consider other languages simulation of unsupported features an error, first of all because the behavior will rarely be exactly the expected one, secondly because our curent programming language may already have something similar to better learn and use.

A new ES5( direction )

As soon as I have written the Pascal example, I have realized that the good "old" Object.defineProperty, implemented in all major browsers (IE < 9 sucks, you know that ...), has basically the same meaning: define object accessors.

The only missing part, intrinsic in the JavaScript nature, is the property type, where this type could be eventually used for arguments or returns checks when the property is a method.

My Early Attempts

Back in May 2007, my JavaStrict experiment was already trying to do something similar, something implemented after 2 years even in dojo framework as well.
This may tell us that "somebody" would really like to put this kind of checks on daily code, even if performances cannot obviously be the best possible one.

Production VS Deployment

Keeping always in mind that performances matter, the new little monster I am going to show here can be flagged with a boolean, nothing different from:

Object.defineStrictProperty.production = true;

Once in place, the whole logic will switch from "strict type" to "free type" and the used function will be the native Object.defineProperty respecting basically the same behavior.

Object.defineStrictProperty / ies

I have put the code in, while I am going to show how it works here.

How to declare a type

Next piece of code is the most basic usage of this method.

var myObject = Object.defineStrictProperty({}, "name", {
type: "string"
}); = "it's a me!";

alert(; = 123;

// throw new TypeError("wrong type");
// console.log =>
// Object
// > expected: "string"
// > received: 123

The meaning of "type"

The type property can contain any of these values:
  • string, where the check will be performed against the typeof value (e.g. "string", "number", "boolean", etc)

  • function, where the check will be performed against the value instanceof type (e.g. Function, Object, Array, Date, etc)

  • object, where the check will be performed against the type.isPrototypeOf(value) (e.g. Array.prototype, others ...)

The only exceptions are null and undefined.

How to declare a returned value

If the property type is "function" or Function there are other extra properties we could define such returns and arguments. Here there is a returns example:

var myObject = Object.defineStrictProperties({}, {
name: {
type: "string",
value: "it's a me!"
toString: {
type: "function",
returns: "string",
value: function () {

alert(myObject); // it's a me!

myObject.toString = function () {
return 456;


// throw new TypeError("wrong return");
// console.log =>
// Object
// > expected: ["string"]
// > received: 456

Multiple returns

Since JavaScript is dynamic, there is nothing wrong, logic a part, into different returns. To make it possible we simply need to specify an array with all expected types.

// precedent code with this difference
toString: {
type: "function",
returns: ["string", "number"],
value: function () {

// precedent example won't fail now

How to declare expected arguments

Similar returns concept applied to arguments, so that we can specify a single argument type, a list of arguments type, a list of arguments. Here the difference:

setName: {
type: "function",
// checks if a single argument
// with typeof string has been sent
arguments: "string",
// same as above
arguments: ["string"],
// checks if two arguments
// with typeof string and number
// has been sent, order matters!
arguments: ["string", "number"],
// checks if one argument
// with typeof string OR number
// has been sent
arguments: [["string"], ["number"]],
value: function (name) { = name;

The number of arguments and relative overload per call is arbitrary. As example, we could have a method defined with these arguments without problems.

// generic object for Ajax calls
get: {
type: "function",
returns: "string",
arguments: [["string"], ["string", "string", "string"]],
value: function (uri, user, pass) {"get", uri, false, pass && user, pass);
return this.responseText;

With a simple logic like that we can "enforce" the program to send one argument or three when, if three, all of them will be checked as strings.

Unlimited arguments

We may decide that the first argument is the only one we would like to check. Well, in this case we can simply create a group of allowed types and ignore the rest.
The arguments logic should be flexible enough due JavaScript nature where overloads can only be simulated via number or tye of arguments checks.

Getters and Setters

The whole logic behind is based on get/set property descriptor, where a nice fallback to __defineGetter__ or __defineSetter__ will be used when Object.defineProperty is missing.
This means that we can describe properties the same way we could do via Object.defineProperty, using enumerable and others as well to make the whole code gracefully degradable when the production flag is set to true.

As Summary

Unless developers won't be interested into this approach, I think this method is basically complete as is and we can use it at own risk.
The whole script stays in about 870 bytes minified and gzipped, not a big deal for what it offers.
In any case, I am looking forward for your feedback!


Brett said...

Very nice. I could see expansion into things like:

1) offering a required arity or arity range for functions

2) regular expressions or functions to validate properties or return types, whether for strings, numbers, or whatever

Andrea Giammarchi said...

thanks Brett. About validation, it's a bit off topic for me but you can already use get/set to filter in/out variables ;)