- JavaScript Basics
- JS Home
- JS Syntax
- JS Placements
- JS Output
- JS Statements
- JS Keywords
- JS Comments
- JS Variables
- JS var
- JS let
- JS const
- JS var Vs let Vs const
- JS Operators
- JS Arithmetic Operators
- JS Assignment Operators
- JS Comparison Operators
- JS Logical Operators
- JS Bitwise Operators
- JS Ternary Operator
- JS Operator Precedence
- JS Data Types
- JS typeof
- JS Conditional Statements
- JS Conditional Statement
- JS if Statement
- JS if...else Statement
- JS switch Statement
- JS Loops
- JS for Loop
- JS while Loop
- JS do...while Loop
- JS break Statement
- JS continue Statement
- JS break Vs. continue
- JavaScript Popup Boxes
- JS Dialog Box
- JS alert Box
- JS confirm Box
- JS prompt Box
- JavaScript Functions
- JS Functions
- JS setTimeout() Method
- JS setInterval() Method
- JavaScript Events
- JS Events
- JS onclick Event
- JS onload Event
- JS Mouse Events
- JS onreset Event
- JS onsubmit Event
- JavaScript Arrays
- JS Array
- JS Find Length of Array
- JS Add Elements at Beginning
- JS Add Element at End
- JS Remove First Element
- JS Remove Last Element
- JS Get First Index
- JS Get Last Index
- JS Reverse an Array
- JS Sort an Array
- JS Concatenate Arrays
- JS join()
- JS toString()
- JS from()
- JS Check if Value Exists
- JS Check if Array
- JS Slice an Array
- JS splice()
- JS find()
- JS findIndex()
- JS entries()
- JS every()
- JS fill()
- JS filter()
- JS forEach()
- JS map()
- JavaScript Strings
- JS String
- JS Length of String
- JS Convert to Lowercase
- JS Convert to Uppercase
- JS String Concatenation
- JS search()
- JS indexOf()
- JS search() Vs. indexOf()
- JS match()
- JS match() Vs. search()
- JS replace()
- JS toString()
- JS String()
- JS includes()
- JS substr()
- JS Slice String
- JS charAt()
- JS repeat()
- JS split()
- JS charCodeAt()
- JS fromCharCode()
- JS startsWith()
- JS endsWith()
- JS trim()
- JS lastIndexOf()
- JavaScript Objects
- JS Objects
- JS Boolean Object
- JavaScript Math/Number
- JS Math Object
- JS Math.abs()
- JS Math.max()
- JS Math.min()
- JS Math.pow()
- JS Math.sqrt()
- JS Math.cbrt()
- JS Math.round()
- JS Math.ceil()
- JS Math.floor()
- JS Math.trunc
- JS toFixed()
- JS toPrecision()
- JS Math.random()
- JS Math.sign()
- JS Number.isInteger()
- JS NaN
- JS Number()
- JS parseInt()
- JS parseFloat()
- JavaScript Date and Time
- JS Date and Time
- JS Date()
- JS getFullYear()
- JS getMonth()
- JS getDate()
- JS getDay()
- JS getHours()
- JS getMinutes()
- JS getSeconds()
- JS getMilliseconds()
- JS getTime()
- JS getUTCFullYear()
- JS getUTCMonth()
- JS getUTCDate()
- JS getUTCDay()
- JS getUTCHours()
- JS getUTCMinutes()
- JS getUTCSeconds()
- JS getUTCMilliseconds()
- JS toDateString()
- JS toLocaleDateString()
- JS toLocaleTimeString()
- JS toLocaleString()
- JS toUTCString()
- JS getTimezoneOffset()
- JS toISOString()
- JavaScript Browser Objects
- JS Browser Objects
- JS Window Object
- JS Navigator Object
- JS History Object
- JS Screen Object
- JS Location Object
- JavaScript Document Object
- JS Document Object Collection
- JS Document Object Properties
- JS Document Object Methods
- JS Document Object with Forms
- JavaScript DOM
- JS DOM
- JS DOM Nodes
- JS DOM Levels
- JS DOM Interfaces
- JavaScript Cookies
- JS Cookies
- JS Create/Delete Cookies
- JavaScript Regular Expression
- JS Regular Expression
- JS RegEx .
- JS RegEx \w and \W
- JS RegEx \d and \D
- JS RegEx \s and \S
- JS RegEx \b and \B
- JS RegEx \0
- JS RegEx \n
- JS RegEx \xxx
- JS RegEx \xdd
- JS RegEx Quantifiers
- JS RegEx test()
- JS RegEx lastIndex
- JS RegEx source
- JavaScript Advance
- JS Page Redirection
- JS Form Validation
- JS Validations
- JS Error Handling
- JS Exception Handling
- JS try-catch throw finally
- JS onerror Event
- JS Multimedia
- JS Animation
- JS Image Map
- JS Debugging
- JS Browser Detection
- JS Security
- JavaScript Misc
- JS innerHTML
- JS getElementById()
- JS getElementsByClassName()
- JS getElementsByName()
- JS getElementsByTagName()
- JS querySelector()
- JS querySelectorAll()
- JS document.write()
- JS console.log()
- JS instanceof
- JavaScript Programs
- JavaScript Programs
JavaScript Variables
Variables in JavaScript are the containers for the data used in the program/application. For example:
<!DOCTYPE html> <html> <body> <p>Sum of <span id="one">10</span> and <span id="two">20</span> is <span id="res"></span></p> <script> var numOne = parseInt(document.getElementById("one").innerHTML); var numTwo = parseInt(document.getElementById("two").innerHTML); var result = numOne + numTwo; document.getElementById("res").innerHTML = result; </script> </body> </html>
Sum of 10 and 20 is
In above example, after initializing the value to variable say numOne, writing numOne anywhere in the program, means writing the value inside it or represents the value stored in it.
Types of Variables in JavaScript
In JavaScript, a variable can be declared using any of the following three keywords:
Depending on the use of keyword to declare a variable in JavaScript, there are three types of variables, that are:
- var Variable
- let Variable
- const Variable
Note - Use var to declare a global variable, that can be updated and re-declared.
Note - Use let to declare a block-scoped variable that can be updated, but can not be re-declared.
Note - Use const to declare a block-level variable that can neither be updated nor be re-declared. A const variable must be initialized with a value at the time of its declaration.
Rules to Name a Variable in JavaScript
- Variable name must starts with a letter (A-Z or a-z), a dollar sign ($), or an underscore (_)
- Use letters, digits (0-9), underscores, and dollar signs to name a variable
- Do not use any keyword to name a variable
- Variable names in JavaScript are case-sensitive. Therefore, num, Num, NUM are all different variables
Here are the list of some valid variable names example in JavaScript:
- num
- x
- _x
- $x
- first_name
- area34
- ar3t
- tr_$s_3_
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