BEST SITE FOR WEB DEVELOPERS

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS String Search JS String Templates JS Numbers JS Number Methods JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS Conditions JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Iterables JS Sets JS Maps JS Typeof JS Type Conversion JS Bitwise JS RegExp JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS IE / Edge JS History

JS Objects

Object Definitions Object Properties Object Methods Object Display Object Accessors Object Constructors Object Prototypes Object Iterables Object Sets Object Maps Object Reference

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Graphics

JS Graphics JS Canvas JS Plotly JS Chart.js JS Google Chart JS D3.js

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor JS Exercises JS Quiz JS Certificate

JS References

JavaScript Objects HTML DOM Objects

Place for your advertisement!
JavaScript. W3Schools in English. Lessons for beginners

Ua

JavaScript Data Types

JavaScript variables can hold different data types: numbers, strings, objects and more:

let length = 16;                               // Number
let lastName = "Johnson";                      // String
let x = {firstName:"John", lastName:"Doe"};    // Object

The Concept of Data Types

In programming, data types is an important concept.

To be able to operate on variables, it is important to know something about the type.

Without data types, a computer cannot safely solve this:

let x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will it produce an error or will it produce a result?

JavaScript will treat the example above as:

let x = "16" + "Volvo";

When adding a number and a string, JavaScript will treat the number as a string.

Example

let x = 16 + "Volvo";
Try it Yourself »

Example

let x = "Volvo" + 16;
Try it Yourself »

JavaScript evaluates expressions from left to right. Different sequences can produce different results:

JavaScript:

let x = 16 + 4 + "Volvo";

Result:

20Volvo
Try it Yourself »

JavaScript:

let x = "Volvo" + 16 + 4;

Result:

Volvo164
Try it Yourself »

In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".

In the second example, since the first operand is a string, all operands are treated as strings.


JavaScript Types are Dynamic

JavaScript has dynamic types. This means that the same variable can be used to hold different data types:

Example

let x;           // Now x is undefined
x = 5;           // Now x is a Number
x = "John";      // Now x is a String
Try it Yourself »

JavaScript Strings

A string (or a text string) is a series of characters like "John Doe".

Strings are written with quotes. You can use single or double quotes:

Example

let carName1 = "Volvo XC60";   // Using double quotes
let carName2 = 'Volvo XC60';   // Using single quotes
Try it Yourself »

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:

Example

let answer1 = "It's alright";             // Single quote inside double quotes
let answer2 = "He is called 'Johnny'";    // Single quotes inside double quotes
let answer3 = 'He is called "Johnny"';    // Double quotes inside single quotes
Try it Yourself »

You will learn more about strings later in this tutorial.


JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:

Example

let x1 = 34.00;     // Written with decimals
let x2 = 34;        // Written without decimals
Try it Yourself »

Extra large or extra small numbers can be written with scientific (exponential) notation:

Example

let y = 123e5;      // 12300000
let z = 123e-5;     // 0.00123
Try it Yourself »

You will learn more about numbers later in this tutorial.


JavaScript Booleans

Booleans can only have two values: true or false.

Example

let x = 5;
let y = 5;
let z = 6;
(x == y)      // Returns true
(x == z)       // Returns false
Try it Yourself »

Booleans are often used in conditional testing.

You will learn more about conditional testing later in this tutorial.


JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars, containing three items (car names):

Example

const cars = ["Saab", "Volvo", "BMW"];
Try it Yourself »

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn more about arrays later in this tutorial.


JavaScript Objects

JavaScript objects are written with curly braces {}.

Object properties are written as name:value pairs, separated by commas.

Example

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

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn more about objects later in this tutorial.


The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.

The typeof operator returns the type of a variable or an expression:

Example

typeof ""             // Returns "string"
typeof "John"         // Returns "string"
typeof "John Doe"     // Returns "string"
Try it Yourself »

Example

typeof 0              // Returns "number"
typeof 314            // Returns "number"
typeof 3.14           // Returns "number"
typeof (3)            // Returns "number"
typeof (3 + 4)        // Returns "number"
Try it Yourself »

You will learn more about typeof later in this tutorial.


Undefined

In JavaScript, a variable without a value, has the value undefined. The type is also undefined.

Example

let car;    // Value is undefined, type is undefined
Try it Yourself »

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Example

car = undefined;    // Value is undefined, type is undefined
Try it Yourself »

Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.

Example

let car = "";    // The value is "", the typeof is "string"
Try it Yourself »

Test Yourself With Exercises

Exercise:

Use comments to describe the correct data type of the following variables:

let length = 16;            // 
let lastName = "Johnson";   // 
const x = {
  firstName: "John",
  lastName: "Doe"
};                          // 


Place for your advertisement!