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
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.
| Value | Type | Comment |
| "Hello" | string | "Hello" is always "Hello" |
| 3.14 | number | 3.14 is always 3.14 |
| true | boolean | true is always true |
| false | boolean | false is always false |
| null | null (object) | null is always null |
| undefined | undefined | undefined is always undefined |
Objects are Variables
JavaScript variables can contain single values:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Variables</h2>
<p id="demo"></p>
<script>
// Create aand display a variable:
var person = "John Doe";
document.getElementById("demo").innerHTML = person;
</script>
</body>
</html>
|
output
JavaScript Variables |
Objects are variables too. But objects can contain many values.
The values are written as name : value pairs (name and value separated by a colon).
Example
<!DOCTYPE html>
<html>
<body>
<p>Creating a JavaScript Object.</p>
<p id="demo"></p>
<script>
var person = {
firstName : "John",
lastName : "Doe",
age : 50,
eyeColor : "blue"
};
document.getElementById("demo").innerHTML =
person.firstName + " " + person.lastName;
</script>
</body>
</html>
|
output
|
Creating a JavaScript Object. John Doe |
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.
Creating a JavaScript Object
With JavaScript, you can define and create your own objects. There are different ways to create new objects:
- Define and create a single object, using an object literal.
- Define and create a single object, with the keyword new.
- Define an object constructor, and then create objects of the constructed type.
In ECMAScript 5, an object can also be created with the function 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
<!DOCTYPE html>
<html>
<body>
<p>Creating a JavaScript Object.</p>
<p id="demo"></p>
<script>
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>
</body>
</html>
|
output
|
Creating a JavaScript Object. John is 50 years old. |
Spaces and line breaks are not important. An object definition can span multiple lines:
Example
<!DOCTYPE html>
<html>
<body>
<p>Creating a JavaScript Object.</p>
<p id="demo"<>/p>
<script>
var person = {
firstName : "John",
lastName : "Doe",
age : 50,
eyeColor : "blue"
};
document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>
</body>
</html>
|
output
|
Creating a JavaScript Object. John is 50 years old. |
Using the JavaScript Keyword new
The following example also creates a new JavaScript object with four properties:
Example
<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
var person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";
document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>
</body>
</html>
|
output
| John is 50 years old. |
The two examples above do exactly the same. There is no need to use new Object(). For simplicity, readability and execution speed, use the first one (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:
| var x = person; // This 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
<!DOCTYPE html>
<html>
<body>
<p>JavaScript objects are mutable.</p>
<p>Any changes to a copy of an object will also change the original.</p>
<p id="demo"></p>
<script>
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"}
var x = person;
x.age = 10;
document.getElementById("demo").innerHTML =
person.firstName + " is " + person.age + " years old.";
</script>
</body>
</html>
|
output
|
JavaScript objects are mutable. Any changes to a copy of an object will also change the original. John is 10 years old. |
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