var vs let vs const

In JavaScript, we often need to store values—like numbers, strings, or objects—for later use. To do that, we declare variables. And JavaScript gives us three keywords to do it: var, let, and const.

But what’s the difference? Which one should you use? Let me explain it like I’m walking you through code in a real project.

Why var Is Old School Now

Back in the early days of JavaScript, var was all we had. It still works today, but it has some quirks that can make your code confusing. For example, variables declared with var are function-scoped. That means they are accessible anywhere inside the function, even before they’re declared (a behavior called hoisting).

				
					function test() {
  console.log(x); // undefined
  var x = 10;
}

This can lead to hard-to-find bugs. Because of these issues, developers now mostly use let and const instead.

When to Use let

Introduced in ES6, let fixes most of the issues var had. Variables declared with let are block-scoped—they only live within the curly braces {} they’re declared in.

Here’s an example:

				
					if (true) {
  let message = "Hello!";
  console.log(message); // Works
}
console.log(message); // ReferenceError: message is not defined

That’s much safer and easier to reason about. If a variable can change (like user input, counters, etc.), use let.

When to Use const

Also added in ES6, const is like let, but with one important rule: you can’t reassign it. Once you set it, it stays that way.

				
					const pi = 3.14;
pi = 3.14159; // ❌error!

This is great for values that should never change, like API keys, fixed configuration values, or constants like pi.

Note: const makes the variable binding constant—not necessarily the value. If it holds an object or array, you can still change the internal contents.

				
					const person = { name: "Arshyan" };
person.name = "Hamza"; // ✅ allowed

Summary

Use let when the value needs to change.
Use const when the value should stay the same.
Avoid var unless you’re maintaining old code.

In modern JavaScript, sticking with let and const keeps your code clean, predictable, and easier to debug. And if you’re unsure, start with const by default—only use let when you know you’ll need to change the value.