CODEWITHSUNDEEP
javascriptfunctionconstructorargumentsabstraction
Joshua Wise
Joshua Wise

Reputation: 653

Dynamically modifying Constructors in JavaScript?

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?

Upvotes: 1

Views: 87

Answers (2)

Felix Kling
Felix Kling

Reputation: 816252

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.

Upvotes: 3

Linial
Linial

Reputation: 1154

Your example looks like a bad practice.

This way it is much easier to maintain,

I'd go like so:

function OriginalConstructor(id, number){

   this.id = id;
   this.number = number;

}
var Obj = new OriginalConstructor();

function newConstructor(Obj){

   this.id = Obj.id;
   this.number = Obj.number * 2;

}

newObject = new newConstructor(Obj);

What you receive from that is: an Obj that was originally created by original constructor, and it's data and state never changed and a newObject which was created by the modifiedConstructor which modifies an object based on a new constructor.

Upvotes: 0

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