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JavaScript Objects


In JavaScript, objects are king. If you understand objects, you understand JavaScript.


In JavaScript, almost "everything" is an object.

  • Booleans can be objects (if defined with the new keyword)
  • Numbers can be objects (if defined with the new keyword)
  • Strings can be objects (if defined with the new keyword)
  • Dates are always objects
  • Maths are always objects
  • Regular expressions are always objects
  • Arrays are always objects
  • Functions are always objects
  • Objects are always objects

All JavaScript values, except primitives, are objects.


JavaScript Primitives

A primitive value is a value that has no properties or methods.

A primitive data type is data that has a primitive value.

JavaScript defines 5 types of primitive data types:

  • string
  • number
  • boolean
  • null
  • undefined

Primitive values are immutable (they are hardcoded and therefore cannot be changed).

if x = 3.14, you can change the value of x. But you cannot change the value of 3.14.

ValueTypeComment
"Hello"string"Hello" is always "Hello"
3.14number3.14 is always 3.14
truebooleantrue is always true
falsebooleanfalse is always false
nullnull (object)null is always null
undefinedundefinedundefined is always undefined

Objects are Variables

JavaScript variables can contain single values:

Example

let person = "John Doe";
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JavaScript variables can also contain many values.

Objects are variables too. But objects can contain many values.

Object values are written as name : value pairs (name and value separated by a colon).

Example

let person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
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A JavaScript object is a collection of named values

It is a common practice to declare objects with the const keyword.

Example

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
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Object Properties

The named values, in JavaScript objects, are called properties.

Property Value
firstName John
lastName Doe
age 50
eyeColor blue

Objects written as name value pairs are similar to:

  • Associative arrays in PHP
  • Dictionaries in Python
  • Hash tables in C
  • Hash maps in Java
  • Hashes in Ruby and Perl

Object Methods

Methods are actions that can be performed on objects.

Object properties can be both primitive values, other objects, and functions.

An object method is an object property containing a function definition.

Property Value
firstName John
lastName Doe
age 50
eyeColor blue
fullName function() {return this.firstName + " " + this.lastName;}

JavaScript objects are containers for named values, called properties and methods.

You will learn more about methods in the next chapters.


Creating a JavaScript Object

With JavaScript, you can define and create your own objects.

There are different ways to create new objects:

  • Create a single object, using an object literal.
  • Create a single object, with the keyword new.
  • Define an object constructor, and then create objects of the constructed type.
  • Create an object using Object.create().

Using an Object Literal

This is the easiest way to create a JavaScript Object.

Using an object literal, you both define and create an object in one statement.

An object literal is a list of name:value pairs (like age:50) inside curly braces {}.

The following example creates a new JavaScript object with four properties:

Example

const person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
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Spaces and line breaks are not important. An object definition can span multiple lines:

Example

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 50,
  eyeColor: "blue"
};
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This example creates an empty JavaScript object, and then adds 4 properties:

Example

const person = {};
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
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Using the JavaScript Keyword new

The following example create a new JavaScript object using new Object(), and then adds 4 properties:

Example

const person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
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The examples above do exactly the same.

But there is no need to use new Object().

For readability, simplicity and execution speed, use the object literal method.


JavaScript Objects are Mutable

Objects are mutable: They are addressed by reference, not by value.

If person is an object, the following statement will not create a copy of person:

const x = person;  // Will not create a copy of person.

The object x is not a copy of person. It is person. Both x and person are the same object.

Any changes to x will also change person, because x and person are the same object.

Example

const person = {
  firstName:"John",
  lastName:"Doe",
  age:50, eyeColor:"blue"
}

const x = person;
x.age = 10;      // Will change both x.age and person.age
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