--- title: Object Definitions description: "> 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) -…" url: "https://apprentice.mn/docs/object-definitions/" updated: "2021-05-18" category: Javascript --- # Object Definitions > 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. | 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 `var person = "John Doe";` 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 `var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};` A JavaScript object is a collection of **named values** --- ## 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: - 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 `var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};` Spaces and line breaks are not important. An object definition can span multiple lines: ### Example `var person = { firstName: "John", lastName: "Doe", age: 50, eyeColor: "blue" };` ## Using the JavaScript Keyword new The following example also creates a new JavaScript object with four properties: ### Example `var person = new Object(); person.firstName = "John"; person.lastName = "Doe"; person.age = 50; person.eyeColor = "blue";` 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 `var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"} var x = person; x.age = 10; // This will change both x.age and person.age` [https://www.addtoany.com/add_to/facebook?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions](https://www.addtoany.com/add_to/facebook?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions)[https://www.addtoany.com/add_to/mastodon?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions](https://www.addtoany.com/add_to/mastodon?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions)[https://www.addtoany.com/add_to/email?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions](https://www.addtoany.com/add_to/email?linkurl=https%3A%2F%2Fapprentice.mn%2Fdocs%2Fobject-definitions%2F&linkname=Object%20Definitions)[https://www.addtoany.com/share](https://www.addtoany.com/share)