Smalltalk Classes in JavaScript

Smalltalk is a class-based language and JavaScript is a prototype-based language.

It would be nice if objects in JavaScript had “class” values, but they don’t.

Try this in Firebug (Firefox plugin), Chrome’s console, or any other JavaScript interactive environment:

(123).class
==>
undefined

Numbers don’t know their class.

So, let’s fix that:

Number.prototype.class = ‘I am a Smalltalk number’

Now try the example above again:

(123).class
==>
‘I am a Smalltalk number’

And it is not just “123” that now has a class. *Every* number now thinks that it is a Smalltalk number.

Math.PI.class
==>
‘I am a Smalltalk number’

(Math.sin(77/92.6)-123.45).class
==>
‘I am a Smalltalk number’

You can do the same for Strings, Arrays, and Functions:

String.prototype.class = ‘I am a Smalltalk string’
Array.prototype.class = ‘I am a Smalltalk array’
Function.prototype.class = ‘I am a Smalltalk block’

Now try these:

‘Smalltalk is better than JavaScript’.class
==>
‘I am a Smalltalk string’

[1,2,3,4].class
==>
‘I am a Smalltalk array’

(_=function(x) {return x*x;}).class
==>
‘I am a Smalltalk block’

Of course, putting a string as the class value isn’t very useful.
In QuickSilver “.class” points to a Smalltalk “class” object.

BTW, don’t do the following:

Object.prototype.class = ‘I am an Object’

Changing the “Object” prototype can mess up a number of JS libraries.

However, you can set “class” for individual objects:

x = {}
typeof(x)
==>
‘object’
x.class
==>
undefined
x.class = ‘I am a blue widget’
x.class
==>
‘I am a blue widget’

In practical Smalltalk terms this means that you can assign the class value while performing a “basicNew” operation to create a new instance.

So, with a little effort, you can make JavaScript objects have class values.