Use the reserved var keyword to declare a variable in JavaScript or you can declare a JavaScript Variable in other ways:
Declare a JavaScript variable with the var or the let keyword:
var name;
let address;There are some rules while declaring a JavaScript variable (known as identifiers).
- The name must start with a letter (a to z or A to Z), underscore( _ ), or dollar( $ ) sign.
- After the first letter, we can use digits (0 to 9), for example, value1.
- JS variables are case-sensitive, for example, x and X are different.
After the declaration, the variable has no value (technically it is undefined).
To assign a value to the variable, use the equal sign:
var name = "ABC";Variable declaration in JavaScript
Simple example code.
<!DOCTYPE html>
<html>
<body>
<script>
var x = 500;
var y = 600;
var z = x + y;
console.log("z variable values:",z)
</script>
</body>
</html>
Output:
JavaScript declares the variable type
JavaScript variables can hold different data types: numbers, strings, objects, and more:
let length = 100; // Number
let lastName = "ABC"; // String
let x = {firstName:"John", lastName:"King"}; // Object Here’s a tabular representation of variable declaration in JavaScript:
| Keyword | Scope | Hoisting | Reassignment | Example |
|---|---|---|---|---|
var | Function/global | Hoisted | Allowed | var x = 10; |
let | Block | Not hoisted | Allowed | let a = 30; if (true) { let b = 40; } |
const | Block | Not hoisted | Not allowed | const PI = 3.14; |
- Scope:
Function/global: Variables are scoped to the function or globally.Block: Variables are scoped to the block ({ }) where they are defined.
- Hoisting:
Hoisted: Variables are moved to the top of their scope during compilation.Not hoisted: Variables are not moved to the top of their scope.
- Reassignment:
Allowed: Variables can be reassigned to a new value.Not allowed: Variables cannot be reassigned after the initial assignment.
Do comment if you have any doubts or suggestions on this JS variable topic.
Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.
OS: Windows 10
Code: HTML 5 Version