constructs

@page Constructs Constructor Functions @parent Tutorial 0

@body Constructor functions made with can.Construct are used to create objects with shared properties, and they make managing inheritance in JavaScript much easier. [Observes] (and through Observes, [Models] and [Controls]) are based off of Constructs, so learning how they work is fundamental to understanding CanJS.

To create a constructor function of your own, call can.Construct and pass in an object of static properties (which will be attached directly to the constructor) and an object of instance properties (which will be attached to each instance of the Construct):

@codestart var Todo = can.Construct({ }, { description: 'Something to do.', author: 'Unknown', allowedToEdit: function() { return true; } });

var t = new Todo();

t.description; // 'Something to do.' t.author; // 'Unknown' t.allowedToEdit(); // true @codeend

There are a few other ways to create classes with can.Construct; see the API for all the details.

Inheritance

can.Construct sets up the prototype chain so that Constructs are easy to subclass. Call the constructor function of the class you want to extend and give it the arguments you would pass to can.Construct:

@codestart // If only one argument is passed, they are considered instance properties. var PrivateTodo = Todo({ description: 'Something secret!', allowedToEdit: function(account) { return account.owns(this); } });

var p = new PrivateTodo(); p.author; // 'Unknown' p.description; // 'Something secret!' p.allowedToEdit({owns: function(){ return false; }}); // false @codeend

Initialization

As you can see above, when a constructor function is called with new, can.Construct creates a new instance of that class. If you've supplied an instance method called init, can.Construct will call that as well, passing it the arguments you passed to the constructor.

This helps make our Todo a little more configurable:

@codestart var Todo = can.Construct({ description: 'Something to do.', author: 'Unknown',

init: function(options) {
	this.author = options.author || this.author;
	this.description = options.description || this.description;
}

});

var t = new Todo({author: 'Me!'}); t.author; // 'Me!' t.description; // 'Something to do.' @codeend

If you're extending a Construct, you probably want to make sure you call the base's init method inside the child's init:

@codestart var PrivateTodo = can.Construct({ init: function(options) { can.Construct.prototype.init.apply(this, arguments); } }); @codeend