---
title: Object Constructors
description: "### Example `function Person(first, last, age, eye) { this.firstName = first; this.lastName = last; this.age = age; this.eyeColor = eye; }` It is considered good practice to name constructor functions with an upper-case first letter. --- ## Object Types (Blueprints)…"
url: "https://apprentice.mn/docs/object-constructors/"
updated: "2021-05-18"
category: Javascript
---

# Object Constructors

### Example

     `function Person(first, last, age, eye) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eye;
}`

It is considered good practice to name constructor functions with an upper-case first letter.

---

## Object Types (Blueprints) (Classes)

The examples from the previous chapters are limited. They only create single objects.

Sometimes we need a “**blueprint**” for creating many objects of the same “type”.

The way to create an “object type”, is to use an **object constructor function**.

In the example above, `function Person()` is an object constructor function.

Objects of the same type are created by calling the constructor function with the `new` keyword:

     `var myFather = new Person("John", "Doe", 50, "blue");
var myMother = new Person("Sally", "Rally", 48, "green");`

## The **this** Keyword

In JavaScript, the thing called `this` is the object that “owns” the code.

The value of `this`, when used in an object, is the object itself.

In a constructor function `this` does not have a value. It is a substitute for the new object. The value of `this` will become the new object when a new object is created.

Note that `this` is not a variable. It is a keyword. You cannot change the value of `this`.

---

## Adding a Property to an Object

Adding a new property to an existing object is easy:

### Example

      `myFather.nationality = "English";`

The property will be added to myFather. Not to myMother. (Not to any other person objects).

---

## Adding a Method to an Object

Adding a new method to an existing object is easy:

### Example

      `myFather.name = function () {
  return this.firstName + " " + this.lastName;
};`

The method will be added to myFather. Not to myMother. (Not to any other person objects).

---

## Adding a Property to a Constructor

You cannot add a new property to an object constructor the same way you add a new property to an existing object:

### Example

      `Person.nationality = "English";`

To add a new property to a constructor, you must add it to the constructor function:

### Example

      `function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
  this.nationality = "English";
}`

This way object properties can have default values.

---

## Adding a Method to a Constructor

Your constructor function can also define methods:

### Example

      `function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
  this.name = function() {return this.firstName + " " + this.lastName;};
}`

You cannot add a new method to an object constructor the same way you add a new method to an existing object.

Adding methods to an object constructor must be done inside the constructor function:

### Example

      `function Person(firstName, lastName, age, eyeColor) {
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
  this.eyeColor = eyeColor;
  this.changeName = function (name) {
    this.lastName = name;
  };
}`

The changeName() function assigns the value of name to the person’s lastName property.

### Now You Can Try:

      `myMother.changeName("Doe");`

avaScript knows which person you are talking about by “substituting” **this** with **myMother**.

---

## Built-in JavaScript Constructors

JavaScript has built-in constructors for native objects:

### Example

      `var x1 = new Object();    // A new Object object
var x2 = new String();    // A new String object
var x3 = new Number();    // A new Number object
var x4 = new Boolean();   // A new Boolean object
var x5 = new Array();     // A new Array object
var x6 = new RegExp();    // A new RegExp object
var x7 = new Function();  // A new Function object
var x8 = new Date();      // A new Date object`

The `Math()` object is not in the list. `Math` is a global object. The `new` keyword cannot be used on `Math`.

---

## Did You Know?

As you can see above, JavaScript has object versions of the primitive data types `String`, `Number`, and `Boolean`. But there is no reason to create complex objects. Primitive values are much faster.

ALSO:

Use object literals `{}` instead of `new Object()`.

Use string literals `""` instead of `new String()`.

Use number literals `12345` instead of `new Number()`.

Use boolean literals `true / false` instead of `new Boolean()`.

Use array literals `[]` instead of `new Array()`.

Use pattern literals `/()/` instead of `new RegExp()`.

Use function expressions `() {}` instead of `new Function()`.

### Example

      `var x1 = {};            // new object
var x2 = "";            // new primitive string
var x3 = 0;             // new primitive number
var x4 = false;         // new primitive boolean
var x5 = [];            // new array object
var x6 = /()/           // new regexp object
var x7 = function(){};  // new function object`

> ## String Objects
>
>
>
> Normally, strings are created as primitives: `var firstName = "John"`
>
>
>
> But strings can also be created as objects using the `new` keyword: `var firstName = new String("John")`
>
>
>
> Learn why strings should not be created as object in the chapter JS Strings.

---

> ## Number Objects
>
>
>
> Normally, numbers are created as primitives: `var x = 123`
>
>
>
> But numbers can also be created as objects using the `new` keyword: `var x = new Number(123)`
>
>
>
> Learn why numbers should not be created as object in the chapter JS Numbers.

---

## Boolean Objects

Normally, booleans are created as primitives: `var x = false`

But booleans can also be created as objects using the `new` keyword: `var x = new Boolean(false)`

Learn why booleans should not be created as object in the chapter JS Booleans.

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