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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 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


JavaScript Syntax

JavaScript syntax is the set of rules, how JavaScript programs are constructed:

// How to create variables:
var x;
let y;

// How to use variables:
x = 5;
y = 6;
let z = x + y;

JavaScript Values

The JavaScript syntax defines two types of values:

  • Fixed values
  • Variable values

Fixed values are called Literals.

Variable values are called Variables.


JavaScript Literals

The two most important syntax rules for fixed values are:

1. Numbers are written with or without decimals:

10.50

1001
Try it Yourself »

2. Strings are text, written within double or single quotes:

"John Doe"

'John Doe'
Try it Yourself »


JavaScript Variables

In a programming language, variables are used to store data values.

JavaScript uses the keywords var, let and const to declare variables.

An equal sign is used to assign values to variables.

In this example, x is defined as a variable. Then, x is assigned (given) the value 6:

let x;
x = 6;
Try it Yourself »

JavaScript Operators

JavaScript uses arithmetic operators ( + - * / ) to compute values:

(5 + 6) * 10
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JavaScript uses an assignment operator ( = ) to assign values to variables:

let x, y;
x = 5;
y = 6;
Try it Yourself »

JavaScript Expressions

An expression is a combination of values, variables, and operators, which computes to a value.

The computation is called an evaluation.

For example, 5 * 10 evaluates to 50:

Expressions can also contain variable values:

The values can be of various types, such as numbers and strings.

For example, "John" + " " + "Doe", evaluates to "John Doe":

"John" + " " + "Doe"
Try it Yourself »

JavaScript Keywords

JavaScript keywords are used to identify actions to be performed.

The let keyword tells the browser to create variables:

let x, y;
x = 5 + 6;
y = x * 10;
Try it Yourself »

The var keyword also tells the browser to create variables:

var x, y;
x = 5 + 6;
y = x * 10;
Try it Yourself »

In these examples, using var or let will produce the same result.

You will learn more about var and let later in this tutorial.


JavaScript Comments

Not all JavaScript statements are "executed".

Code after double slashes // or between /* and */ is treated as a comment.

Comments are ignored, and will not be executed:

let x = 5;   // I will be executed

// x = 6;   I will NOT be executed
Try it Yourself »

You will learn more about comments in a later chapter.


JavaScript Identifiers

Identifiers are names.

In JavaScript, identifiers are used to name variables (and keywords, and functions, and labels).

The rules for legal names are much the same in most programming languages.

In JavaScript, the first character must be a letter, or an underscore (_), or a dollar sign ($).

Subsequent characters may be letters, digits, underscores, or dollar signs.

Numbers are not allowed as the first character.
This way JavaScript can easily distinguish identifiers from numbers.


JavaScript is Case Sensitive

All JavaScript identifiers are case sensitive

The variables lastName and lastname, are two different variables:

let lastname, lastName;
lastName = "Doe";
lastname = "Peterson";
Try it Yourself »

JavaScript does not interpret LET or Let as the keyword let.


JavaScript and Camel Case

Historically, programmers have used different ways of joining multiple words into one variable name:

Hyphens:

first-name, last-name, master-card, inter-city.

Hyphens are not allowed in JavaScript. They are reserved for subtractions.

Underscore:

first_name, last_name, master_card, inter_city.

Upper Camel Case (Pascal Case):

FirstName, LastName, MasterCard, InterCity.

Lower Camel Case:

JavaScript programmers tend to use camel case that starts with a lowercase letter:

firstName, lastName, masterCard, interCity.


JavaScript Character Set

JavaScript uses the Unicode character set.

Unicode covers (almost) all the characters, punctuations, and symbols in the world.

For a closer look, please study our Complete Unicode Reference.