JavaScript Patterns 3.2 Custom Constructor Functions

When you invoke the constructor function with new, the following happens inside the function:

• An empty object is created and referenced by this variable, inheriting the prototype of the function.

• Properties and methods are added to the object referenced by this.

• The newly created object referenced by this is returned at the end implicitly (if no other object was returned explicitly).

var Person = function (name) {

    this.name = name;

    this.say = function () {

        return "I am " + this.name;

    };

};   

var adam = new Person("Adam");

adam.say(); // "I am Adam" 

Note

reusable members, such as methods, should go to the prototype.

Person.prototype.say = function () {

    return "I am " + this.name;

};

Constructor's Return Values

When invoked with new, a constructor function always returns an object inheriting from the constructor's prototype.

var Objectmaker = function () {

    // this `name` property will be ignored

    // because the constructor

    // decides to return another object instead

    this.name = "This is it";

    // creating and returning a new object

    var that = {};

    that.name = "And that's that";

    return that;

};

// test

var o = new Objectmaker();

console.log(o.name); // "And that's that"   

You have the freedom to return any object in your constructors, as long as it's an object. Attempting to return something that's not an object (like a string or a boolean false, for example) will not cause an error but will simply be ignored, and the object referenced by this will be returned instead.