Constructor Functions

Constructor functions are used by JavaScript to create objects with shared properties. It's similar in concept to a class. Constructor functions can be created by using can.Construct. Constructs are used to create instantiable objects with shared properties and help make managing inheritance in JavaScript easier. Observables, Models & Retrieving Data and Controls are based off of Constructs, so learning how they work is fundamental to understanding CanJS.

To create a Construct of your own, call can.Construct.extend with an object of static properties (which will be attached directly to the constructor object) along with an object of instance properties (which will be attached to the constructor's prototype):

var Todo = can.Construct.extend({}, {

  isSecret: function(){
    return false;
  }
  allowedToEdit: function() {
    return ! this.isSecret();
  }
});

var t = new Todo();
t.allowedToEdit(); // true

Inheritance

can.Construct automatically sets up the prototype chain so that Constructs are easy to extend (similar to sub classing). To extend one of your Constructs, call the constructor's extend method, passing it the same arguments that you would pass to can.Construct.extend:

// If only one argument is passed, they are considered prototype properties.
var PrivateTodo = Todo.extend({},{

  isSecret: function() {
    return true;
  }
});

var p = new PrivateTodo();
p.allowedToEdit(); // false

Initialization

When a constructor function is called with new, can.Construct creates a new instance of that class. If you've supplied an prototype method called init, can.Construct will call init with this as the new instance and the arguments passed to the constructor.

This helps make our Todo configurable:

var Todo = can.Construct.extend({
  init: function(owner) {
    this.owner = owner;
  },
  allowedToEdit: function() {
    return true;
  }
});

var t = new Todo("me");
t.owner; // 'me'

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

var PrivateTodo = can.Construct.extend({
  init: function(owner, isShared) {
    can.Construct.prototype.init.apply(this, arguments);
    this.isShared = isShared;
  },
  allowedToEdit: function(){
    return this.owner === "me" || this.isShared;
  }
});

If you find yourself using inheritence a lot, checkout can.Construct's super plugin.