Tutorials References Exercises Videos Menu
Create Website Get Certified Upgrade

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 BigInt JS Number Methods JS Number Properties 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 If Else 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 Precedence JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS Modules 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 2019 JS 2020 JS 2021 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 Bind 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


JavaScript Variables

4 Ways to Declare a JavaScript Variable:

  • Using var
  • Using let
  • Using const
  • Using nothing

What are Variables?

Variables are containers for storing data (storing data values).

In this example, x, y, and z, are variables, declared with the var keyword:

Example

var x = 5;
var y = 6;
var z = x + y;
Try it Yourself »

In this example, x, y, and z, are variables, declared with the let keyword:

Example

let x = 5;
let y = 6;
let z = x + y;
Try it Yourself »

In this example, x, y, and z, are undeclared variables:

Example

x = 5;
y = 6;
z = x + y;
Try it Yourself »

From all the examples above, you can guess:

  • x stores the value 5
  • y stores the value 6
  • z stores the value 11

When to Use JavaScript var?

Always declare JavaScript variables with var,let, orconst.

The var keyword is used in all JavaScript code from 1995 to 2015.

The let and const keywords were added to JavaScript in 2015.

If you want your code to run in older browsers, you must use var.


When to Use JavaScript const?

If you want a general rule: always declare variables with const.

If you think the value of the variable can change, use let.

In this example, price1, price2, and total, are variables:

Example

const price1 = 5;
const price2 = 6;
let total = price1 + price2;
Try it Yourself »

The two variables price1 and price2 are declared with the const keyword.

These are constant values and cannot be changed.

The variable total is declared with the let keyword.

This is a value that can be changed.


Just Like Algebra

Just like in algebra, variables hold values:

let x = 5;
let y = 6;

Just like in algebra, variables are used in expressions:

let z = x + y;

From the example above, you can guess that the total is calculated to be 11.

Note

Variables are containers for storing values.



JavaScript Identifiers

All JavaScript variables must be identified with unique names.

These unique names are called identifiers.

Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).

The general rules for constructing names for variables (unique identifiers) are:

  • Names can contain letters, digits, underscores, and dollar signs.
  • Names must begin with a letter.
  • Names can also begin with $ and _ (but we will not use it in this tutorial).
  • Names are case sensitive (y and Y are different variables).
  • Reserved words (like JavaScript keywords) cannot be used as names.

Note

JavaScript identifiers are case-sensitive.


The Assignment Operator

In JavaScript, the equal sign (=) is an "assignment" operator, not an "equal to" operator.

This is different from algebra. The following does not make sense in algebra:

x = x + 5

In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.

(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)

Note

The "equal to" operator is written like == in JavaScript.


JavaScript Data Types

JavaScript variables can hold numbers like 100 and text values like "John Doe".

In programming, text values are called text strings.

JavaScript can handle many types of data, but for now, just think of numbers and strings.

Strings are written inside double or single quotes. Numbers are written without quotes.

If you put a number in quotes, it will be treated as a text string.

Example

const pi = 3.14;
let person = "John Doe";
let answer = 'Yes I am!';
Try it Yourself »

Declaring a JavaScript Variable

Creating a variable in JavaScript is called "declaring" a variable.

You declare a JavaScript variable with the var or the let keyword:

var carName;
or:
let carName;

After the declaration, the variable has no value (technically it is undefined).

To assign a value to the variable, use the equal sign:

carName = "Volvo";

You can also assign a value to the variable when you declare it:

let carName = "Volvo";

In the example below, we create a variable called carName and assign the value "Volvo" to it.

Then we "output" the value inside an HTML paragraph with id="demo":

Example

<p id="demo"></p>

<script>
let carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>
Try it Yourself »

Note

It's a good programming practice to declare all variables at the beginning of a script.


One Statement, Many Variables

You can declare many variables in one statement.

Start the statement with let and separate the variables by comma:

Example

let person = "John Doe", carName = "Volvo", price = 200;
Try it Yourself »

A declaration can span multiple lines:

Example

let person = "John Doe",
carName = "Volvo",
price = 200;
Try it Yourself »

Value = undefined

In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.

A variable declared without a value will have the value undefined.

The variable carName will have the value undefined after the execution of this statement:

Example

let carName;
Try it Yourself »

Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable declared with var, it will not lose its value.

The variable carName will still have the value "Volvo" after the execution of these statements:

Example

var carName = "Volvo";
var carName;
Try it Yourself »

Note

You cannot re-declare a variable declared with let or const.

This will not work:

let carName = "Volvo";
let carName;

JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

Example

let x = 5 + 2 + 3;
Try it Yourself »

You can also add strings, but strings will be concatenated:

Example

let x = "John" + " " + "Doe";
Try it Yourself »

Also try this:

Example

let x = "5" + 2 + 3;
Try it Yourself »

Note

If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.

Now try this:

Example

let x = 2 + 3 + "5";
Try it Yourself »

JavaScript Dollar Sign $

Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:

Example

let $ = "Hello World";
let $$$ = 2;
let $myMoney = 5;
Try it Yourself »

Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library.

In the JavaScript library jQuery, for instance, the main function $ is used to select HTML elements. In jQuery $("p"); means "select all p elements".


JavaScript Underscore (_)

Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:

Example

let _lastName = "Johnson";
let _x = 2;
let _100 = 5;
Try it Yourself »

Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for "private (hidden)" variables.


Test Yourself With Exercises

Exercise:

Create a variable called carName and assign the value Volvo to it.

var  = "";

Start the Exercise