How do I wrap a constructor?

Question

I have this JavaScript:

var Type = function(name) {
    this.name = name;
};

var t = new Type();

Now I want to add this:

var wrap = function(cls) {
    // ... wrap constructor of Type ...
    this.extraField = 1;
};

So I can do:

wrap(Type);
var t = new Type();

assertEquals(1, t.extraField);

[EDIT] I'd like an instance property, not a class (static/shared) property.

The code executed in the wrapper function should work as if I had pasted it into the real constructor.

The type of Type should not change.

Tiruchirapalli

By what I understand, you want to add an additional property to the constructor? so that further new instances have that property? – Involucrum

May be you need simply to change prototype of Type inside wrap() function. For example: var wrap = function(cls) { cls.prototype.extraField=1; };? Or may be is best to create new Type2 inherited from Type with additional extraField member? – Megrim

Can you describe your issue more detail? – Teatime

I'd like an instance property, not a class (static/shared) property. The code executed in the wrapper function should work as if I had pasted it into the real constructor. – Tiruchirapalli

Answer 1

update: An updated version here

what you were actually looking for was extending Type into another Class. There are a lot of ways to do that in JavaScript. I'm not really a fan of the new and the prototype methods of building "classes" (I prefer the parasitic inheritance style better), but here's what I got:

//your original class
var Type = function(name) {
    this.name = name;
};

//our extend function
var extend = function(cls) {

    //which returns a constructor
    function foo() {

        //that calls the parent constructor with itself as scope
        cls.apply(this, arguments)

        //the additional field
        this.extraField = 1;
    }

    //make the prototype an instance of the old class
    foo.prototype = Object.create(cls.prototype);

    return foo;
};

//so lets extend Type into newType
var newType = extend(Type);

//create an instance of newType and old Type
var t = new Type('bar');
var n = new newType('foo');


console.log(t);
console.log(t instanceof Type);
console.log(n);
console.log(n instanceof newType);
console.log(n instanceof Type);

Involucrum

From your console output, I'd like to extend the function stored in constructor. The new field should show up next to name, not next to constructor. – Tiruchirapalli

so what you really want to do is create another constructor? or just add to the existing? – Involucrum

I want to extend the existing constructor. – Tiruchirapalli

instead of cls.prototype.constructor.call(this, name), I'd do cls.call(this, name). Same function, and no need to worry if the .constructor property is actually present. The danger of foo.prototype = new cls(); is that cls may be a function that requires arguments. That's where foo.prototype = Object.create(cls.prototype) is preferred. – Subsellium

@amnotiam interesting. Now I learned something new. But Object.create is ECMA5, how would you do this in older browsers? – Involucrum

this example shows the classic fix. It only emulates the "object creation with prototype" part of Object.create, but that's all you need here. – Subsellium