10.1 Object Definitions

Objects are Variables

JavaScript variables can contain single values:

Example 1: JS Variables - Objects are Variables part 1

let person = "John Doe";

• 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 2: JS Variables - Objects are Variables part 2

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

• A JavaScript object is a collection of named values
• It is a common practice to declare objects with the const keyword.

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 3: JS Objects - Using an Object Literal part 1

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

Spaces and line breaks are not important. An object definition can span multiple lines:

Example 4: JS Objects - Using an Object Literal part 2

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

This example creates an empty JavaScript object, and then adds 4 properties:

Example 5: JS Objects - Using an Object Literal part 3

const person = {};
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";

Using the JavaScript Keyword new

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

Example 6: JS Objects - Using the Keyword new

const person = new Object();
person.firstName = "John";
person.lastName = "Doe";
person.age = 50;
person.eyeColor = "blue";

• 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 7: JS Objects are Mutable

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

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