Dynamically modifying Constructors in JavaScript?

Question

I'm looking to do something a little bit fancy with constructor functions in Javascript, and I'm not quite sure how to do it.

I want to be able to define Constructor functions, and then pass them into another function (a "modifer") like this:

function OriginalConstructor() {
    // Do constructor things here
    // Like defining methods and properties
}

NewConstructor = modifyConstructor(OriginalConstructor);

And the resulting "NewConstructor" should be functionally equivalent to this:

function NewConstructor(id, data) {
    this.id = id;
    this.data = data;
    // Do stuff from the original constructor here
    // i.e. the same methods and properties defined in the original constructor
}

Does anybody know how to go about creating the "modifyConstructor" function?

Answer 1

You create a function that sets the properties as you defined and calls the original constructor. For example:

function modifyConstructor(Constr) {
    function NewConstructor(id, data) {
        this.id = id;
        this.data = data;

        // Call original constructor
        Constr.apply(this, Array.prototype.slice.call(arguments, 2));
    }
    // Setting the constructor property might not be a good idea depending on
    // the use case
    NewConstructor.prototype = Object.create(Constr.prototype, {
        constructor: {value: NewConstructor, writable: true, configurable: true}
    });
    return NewConstructor;
}

This basically implements NewConstructor as a subclass of whatever Constr is.