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JavaScript

Abe Layee
Abe Layee
8,378 Points

I am lost with instance and constructor object.

I am taking the JavaScript Object cousre and I have been doing good till I got to constructor object. Can anyone please explain to me what Instance and constructor objects are or what is instance in object? Thank you in advance. I copied these codes from the quiz. I have no idea how to identify which one is object or instance

function Monster(rank) { //constructor function
  this.rank = rank;
  this.health = 100;
  this.takeHit = function() {
    this.health--;
  }
  this.isDead = function() {
    return this.health <= 0;
  }
}

var monster = new Monster("Captain");

This is an instance which I copied from the challenge

function TodoListItem(description) {
  this.description = description;
  this.isDone = false;
  this.toString = function() {
    return this.description;
  }
  this.markAsComplete = function() {
    this.isDone = true;
  }
}

var task = new TodoListItem("Do the laundry!");

4 Answers

Hi Abraham,

Great question. First thing to get clear is that objects are new instances created by a constructor. Said another way, objects are instances, and they come from constructors. You can use object and instance interchangeably.

One way that helps me understand the difference is that a constructor is like a blueprint - it defines what a thing is. An object, is the actual thing built from the definition provided by a constructor.

Consider a house blueprint. That's a constructor that defines how to build a particular home. An actual house built using this blueprint would be a real object that someone could move into.

// constructor that creates new objects/instances
function House() {
  var sqft = 1350;
  var numFloors = 2;
  var sizeOfFrontYard = 540;
  var sizeOfBackYard = 720;
  var exteriorColorScheme = { primary : 'gray', secondary : 'white' };
}

// house is a new object/instance created by the House constructor
var house = new House();

I hope this helps. Please let me know if you're still stuck and I'll figure out a way to help guide you closer to understanding this tricky concept.

Cheers!

Abe Layee
Abe Layee
8,378 Points

My question is the last line of the code where you called the House() function or assign it to the House var. I know we use the new word to define a constructor object and your house() does not have. In this case, the House() is the instance and the last line of your code is the object? Right or not? I am sorry if you can't understand what I am trying to say here. I am just trying to wrap my head around it and thank again Robert Richey

// constructor that creates new objects/instances
function House() {
  var sqft = 1350;
  var numFloors = 2;
  var sizeOfFrontYard = 540;
  var sizeOfBackYard = 720;
  var exteriorColorScheme = { primary : 'gray', secondary : 'white' };
}

// house is a new object/instance created by the House constructor
var house = new House(); // shoudln't this call the House() function/instaces

No worries, I know this is a confusing topic.

Instead of saying var house = new House();, we can call the new object/instance anything. Let's instead call it bacon, because (almost) everyone loves bacon, and it doesn't matter what we call each new instance created by the constructor.

// bacon is a new object created by the House constructor
// bacon is a new instance created by the House constructor
// see how object and instance are interchangeable?

var bacon = new House(); // the House() constructor is used to create a new object, called bacon
bacon.numFloors; // the bacon object is getting the property numFloors whose value is 2

// if we want to see this value, we might send it to the console.log() function
console.log(bacon.numFloors); // 2
Jeremy Fisk
Jeremy Fisk
7,768 Points

The constructor function provides attributes and methods to an instance of an object that you create.

In your example, var monster is an instance of the Monster object, and the constructor function is a way of pre-defining what the Monster object will consist of, upon instantiation through your code, :

var monster = new Monster("Captain");

so, since you defined that rank, health, and the two methods will be part of your Monster object, anytime you create a Monster from that point forward, it will consist of the attributes and functions present in your constructor function

Abe Layee
Abe Layee
8,378 Points

aww thank, I think I undertand it now. I really appreciate your help Robert Richey .

To Abraham,

To further clarify an instance is a single occurrence of something. In this case it is an object ( var house ) , built using the blue print of the constructor function ( House).

This enables you to create many different instances of the same type of object. All though each instance of the object will have the same properties and methods, they will more than likely have different values attached to their properties.