How to Declare Variables in JavaScript: Syntax and Examples

In JavaScript, you declare variables using let, const, or var keywords followed by the variable name. let and const are modern ways to declare variables, where const creates a constant value and let allows reassignment.

Syntax

JavaScript variables can be declared using three keywords:

• let: Declares a block-scoped variable that can be reassigned.
• const: Declares a block-scoped constant that cannot be reassigned.
• var: Declares a function-scoped variable (older style, less recommended).

The general syntax is: keyword variableName = value; where keyword is one of let, const, or var.

javascript

let name = 'Alice';
const age = 30;
var city = 'New York';

Example

This example shows how to declare variables with let, const, and var, and how their values can be used or changed.

javascript

let score = 10;
score = 15; // Reassigned

const maxScore = 20;
// maxScore = 25; // This would cause an error

var level = 1;
level = 2;

console.log('Score:', score);
console.log('Max Score:', maxScore);
console.log('Level:', level);

Output

Score: 15
Max Score: 20
Level: 2

Common Pitfalls

Common mistakes when declaring variables include:

• Using const but trying to reassign the variable, which causes an error.
• Using var can lead to unexpected behavior due to its function scope and hoisting.
• Forgetting to declare a variable (missing keyword) creates a global variable unintentionally.

javascript

/* Wrong: Reassigning const */
const pi = 3.14;
// pi = 3.1415; // Error: Assignment to constant variable

/* Wrong: Missing declaration keyword */
function test() {
  x = 5; // Creates global variable unintentionally
}

test();
console.log(x); // 5

/* Right: Use let or const */
let x = 5;
x = 10; // Allowed
console.log(x); // 10

Output

5
10

Quick Reference

Keyword	Scope	Reassignable	Use Case
let	Block	Yes	General variable declaration
const	Block	No	Constants that do not change
var	Function	Yes	Legacy code, avoid in new code

Key Takeaways

Use let for variables that will change and const for constants.

Avoid var because it has confusing scope and hoisting behavior.

Always declare variables with let, const, or var to avoid creating global variables.

const variables cannot be reassigned after declaration.

Block scope means variables declared with let or const exist only inside the nearest curly braces.