Js Prototypes And Constructors

Objects in JavaScript generally refer to the Prototype/Instance object model. These are very similar to other object oriented languages (like Ruby), but the inner mechanics are also fundamentally different.

Types in JS

Before we get into objects, we need to briefly reiterate the 5 different kinds of non-object types.

There are 5 specific non-object types in Javascript: * 3 of the 5 types can be wrapped in object types - These are: boolean, number, and string * 2 of the 5 types have no specific value - These are: null and undefined - Null indicates that a variable has been declared, but doesn't have a value - Undefined indicates that a variable has not been declared in the current scope

Accessing Properties in JS

There are a couple different ways we can access a property of an object. Instead of explaining them explicitly, I'm just going to list them below:

var objWithProps = { color: 'red', name: 'Ian' };

// first method
var objColor = objWithProps.color;
//=> "red"

// second method
var objColor = objWithProps["color"];
//=> "red"

Constructors and Prototypes

In JavaScript, the concept of a class doesn't exist like it does in Ruby.


What we do have, though, are JavaScript objects. They consist of a constructor and a prototype.

What is a constructor? A constructor, simply put, is a function that is used as a constructor, meaning that it is a function that will be built upon with additional functionality (through prototypes). The only thing that makes a constructor a constructor function is the fact that the function's name is capitalized. This is a convention for constructors in JavaScript. There's no magic power associated with a function name that's capitalized - it's just simply a convention.

function Robot(){

var robot = new Robot();

The code above accomplishes three things:

  • We've instantiated a new object.
    • var robot = new Robot();
  • We've set the object robot equal to a new instance of the constructor Robot.

    robot.constructor == Robot // true
    robot instanceof Robot // true
  • Finally, the robot object will delegate to the Robot.prototype.

    • A function is a unique kind of object in Javascript, and has properties. Functions, by default, get a prototype property. It is always an empty hash. A prototype property is empty when a Javascript object is initialized, and you can add properties to the prototype in 2 ways:
    function Robot() {
    // prototype property definition - method 1
    Robot.prototype.dance = function(){ 
      console.log("Harder, Better, Faster, Stronger.") 
    var daftPunkRobot = new Robot();
    // this will print the string "Harder, Better, Faster, Stronger." to the console
    • When an object is constructed (via the new keyword), it automatically inherits all of the properties of the object constructor's prototype.
    function Monster(){
    Monster.prototype.swimAbility = 10;
    var kaiju = new Monster();
    // => 10
    • What does it mean when an object delegates to a prototype for a constructor? It simply just means that, if a property does not yet exist for a specific object monster, it will simply give those properties to the object based on how that property is defined on the prototype. So when a monster object is instantiated, and a property swimAbility doesn't exist yet on instantiation, it will be given that property from Monster.prototype.


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