How to Declare a Variable in JavaScript

What you’ll build or solve

You’ll declare variables that store values correctly and follow modern JavaScript best practices.

When this approach works best

Use this approach when you:

• Store user input, configuration values, or calculation results.
• Track state inside loops or conditionals.
• Define constants like API URLs or tax rates.

Prefer modern syntax in new projects. Avoid var unless maintaining older codebases.

Prerequisites

• A .js file or an HTML file with a <script> tag
• Basic knowledge of strings, numbers, and arrays

No additional setup is required.

Step-by-step instructions

1) Choose the right declaration keyword

JavaScript has three keywords for declaring variables. Pick the one that matches your intent.

Option A: Use const by default

Use const for values that should not be reassigned.

consttaxRate=0.2;
constname="Alex";

Reassignment causes an error:

consttaxRate=0.2;
// taxRate = 0.3; // Error

Use const unless you know the value must change.

Option B: Use let when reassignment is required

Use let if the variable’s value will change later.

letscore=10;
score=20;

You can also declare first and assign later:

lettotal;
total=100;

Switch to let only when reassignment is needed.

Option C: Use var only for legacy code

var works differently from let and const.

varcount=5;

Avoid var in modern development because it does not respect block scope.

What to look for

Declare and assign in one line:

constuser="Sam";

Or declare first, assign later:

letresult;
result=42;

let and const are block-scoped:

if (true) {
constmessage="Hello";
console.log(message);
}

// console.log(message); // Error

var ignores block scope:

if (true) {
vartest="Visible outside";
}

console.log(test);

You cannot redeclare let or const in the same scope:

letprice=100;
// let price = 200; // Error

const prevents reassignment, not mutation:

constuser= { name:"Sam" };
user.name="Alex";// Allowed

Declare variables before using them to avoid temporal dead zone errors:

lettotal=50;
console.log(total);

Examples you can copy

Example 1: Counter that changes

letcounter=0;

counter+=1;
counter+=1;

console.log(counter);

Use let because the value updates.

Example 2: Configuration value

constAPI_URL="https://api.example.com";

console.log("Fetching from:",API_URL);

Use const for values that should stay fixed.

Example 3: Loop variable

for (leti=0;i<3;i++) {
console.log(i);
}

// console.log(i); // Error

If you use var instead:

for (vari=0;i<3;i++) {
console.log(i);
}

console.log(i);// Accessible here

This difference affects how your code behaves.

Common mistakes and how to fix them

Mistake 1: Reassigning a const variable

What you might do:

constrate=0.2;
rate=0.3;

Why it breaks: const variables cannot be reassigned.

Fix:

letrate=0.2;
rate=0.3;

Mistake 2: Using var inside blocks

What you might do:

if (true) {
varmessage="Hi";
}

Why it breaks: var ignores block scope and may leak outside the block.

Fix:

if (true) {
letmessage="Hi";
}

Mistake 3: Accessing before declaration

What you might do:

console.log(total);
lettotal=50;

Why it breaks: let and const cannot be used before declaration.

Fix:

lettotal=50;
console.log(total);

Troubleshooting

If you see “Assignment to constant variable,” change const to let.

If you see “Cannot access before initialization,” move the declaration above usage.

If a variable is accessible outside a block unexpectedly, check for var.

If changes do not appear in the browser, refresh the page.

If Node shows errors, confirm you are running the correct file.

Quick recap

• Use const by default.
• Use let when reassignment is needed.
• Avoid var in modern code.
• Remember block scope rules.
• Declare variables before using them.