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JS ObjectsLesson #55

JavaScript Object Constructors

Sometimes we need to create many objects of the same type.

Object Constructor Functions

Sometimes we need to create many objects of the same type.

To create an object type we use an object constructor function.

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

Object Type Person

Example Code

javascript

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function Person(first, last, age, eye) {

  this.firstName = first;

  this.lastName = last;

  this.age = age;

  this.eyeColor = eye;

}

In the constructor function, this has no value. > The value of this will become the new object when > a new object is created. > Read More: > JavaScript Object this.

Now we can use new Person() to create many new Person objects:

Example

Example Code

javascript

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const myFather = new Person("John", "Doe", 50, "blue");

const myMother = new Person("Sally", "Rally", 48, "green");

const mySister = new Person("Anna", "Rally", 18, "green");

const mySelf = new Person("Johnny", "Rally", 22, "green");

Property Default Values

A value given to a property will be a default value for all objects created by the constructor:

Example

Example Code

javascript

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function Person(first, last, age, eyecolor) {

this.firstName = first;

this.lastName = last;

this.age = age;

  this.eyeColor = eyecolor;
  this.nationality = "English";

}

Adding a Property to an Object

Adding a property to a created object is easy:

Example

Example Code

javascript

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myFather.nationality = "English";

The new property will be added to myFather. Not to any other Person Objects.

Adding a Property to a Constructor

You can NOT add a new property to an object constructor:

Example

Example Code

javascript

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Person.nationality = "English";

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

Example

Example Code

javascript

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Person.prototype.nationality = "English";

Constructor Function Methods

A constructor function can also have methods:

Example

Example Code

javascript

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function Person(first, last, age, eyecolor) {

this.firstName = first;

this.lastName = last;

this.age = age;

this.eyeColor = eyecolor;
  this.fullName = function() {

    return this.firstName + " " + this.lastName;

  };

}

Adding a Method to an Object

Adding a method to a created object is easy:

Example

Example Code

javascript

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myMother.changeName = function (name) {

  this.lastName = name;

}

The new method will be added to myMother. Not to any other Person Objects.

Adding a Method to a Constructor

You cannot add a new method to an object constructor function.

This code will produce a TypeError:

Example

Example Code

javascript

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Person.changeName = function (name) {

  this.lastName = name;

}

myMother.changeName("Doe");

Adding a new method to a constructor must be done to the constructor function prototype:

Example

Example Code

javascript

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Person.prototype.changeName = function (name) {

  this.lastName = name;

}

myMother.changeName("Doe");

The changeName() function assigns the value of name > to the person's lastName property, substituting this with myMother.

Built-in JavaScript Constructors

JavaScript has built-in constructors for all native objects:

Example

Example Code

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new Object()   // A new Object object

new Array()    // A new Array object

new Map()      // A new Map object

new Set()      // A new Set object

new Date()     // A new Date object

new RegExp()   // A new RegExp object

new Function() // A new Function 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?

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

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

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

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

Example

Example Code

javascript

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"";           // primitive string

0;            // primitive number

false;        // primitive boolean

{};           // object object

[];           // array object

/()/          // regexp object

function(){}; // function

Try it Yourself

JavaScript syntax and algorithm demonstration.

Person.prototype.nationality = "English";

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