JavaScript for PHP Developers

Chapter 4. Object-Oriented Programming

This chapter discusses the object-oriented features of JavaScript, including objects, constructor functions, and prototypes. It also talks about code reuse and inheritance.

Constructors and Classes

In PHP, if you have a Dog class, you create a $fido instance of this class using:

// PHP
$fido = new Dog();

JavaScript has a similar syntax:

// JavaScript
var fido = new Dog();

One important difference is that Dog is not a class in JavaScript because there are no classes in the language. Dog is just a function. But functions that are meant to create objects are called constructor functions.

Syntactically, there is no difference between a regular function and a constructor function. The difference is in the intent. Thus, for readability purposes, it’s a common convention to capitalize the first letter in constructor function names.

When you call a function with the new operator, it always returns an object. The object is known as this inside the body of the function. That’s what happens even if you don’t do anything special in the function. Remember that otherwise (when called without new) every function without an explicit return returns undefined:

function Dog() {
  this.name = "Fido";
  this.sayName = function () {
    return "Woof! " + this.name;
  };
}

var fido = new Dog();
fido.sayName(); // "Woof! Fido"

Note

In JavaScript, just as in PHP, parentheses are optional when you’re not passing arguments to the constructor, so this is also valid: var fido = new Dog;

If ...