🧭Beginner’s (and Not-So-Beginner’s) Guide #3 — const vs let (and a bit about var): What Developers Keep Getting Wrong

#javascript #webdev #beginners #programming

If you’re learning JavaScript, one of the first questions you’ll face is:
👉 Should I use const or let?
And what’s up with this old thing called var?

It sounds simple, right?

But trust me — as a senior dev who’s done countless code reviews and technical interviews, I can tell you:

this is one of those “obvious” things that many developers still get wrong. 😅

So let’s break it down — clearly, practically, and once and for all. 🚀

🔒 `const` — constant, but not always frozen

When you use const, you’re saying:

“This variable name will always refer to the same thing.”

Example:

const name = "Alice";
console.log(name); // Alice

name = "Bob"; // ❌ TypeError

You can’t reassign a const.
But here’s a fun twist: if it’s an object or array, you can still change its contents 👇

const numbers = [1, 2, 3];
numbers.push(4); // ✅ works
console.log(numbers); // [1, 2, 3, 4]

numbers = [0]; // ❌ can't reassign

💡 const doesn’t make the value itself immutable — only the reference (the “box label”) is fixed.
If you truly need immutability, use Object.freeze(), but note it’s shallow.

✅ Use const by default.
It keeps your code predictable and self-documenting — if something shouldn’t change, make it const.

🔁 `let` — for things that change

When you do need to reassign a variable, go for let:

let counter = 0;
counter = counter + 1;
console.log(counter); // 1

You can also use let inside blocks (if, for, {}):

if (true) {
  let message = "Hello!";
  console.log(message); // works here
}

console.log(message); // ❌ ReferenceError

That’s because let is block-scoped — it only exists inside the { } where it was declared.

🧩 Bonus fact:
Both let and const are hoisted, but they live in the Temporal Dead Zone (TDZ) until the code reaches their declaration.
That’s why accessing them too early throws a ReferenceError:

console.log(x); // ❌ ReferenceError
let x = 5;

🚫 `var` — the old-school way (for history lovers)

Before 2015, JavaScript only had var.
It kind of works like let, but with some weird, legacy behaviors.

Example:

if (true) {
  var name = "Charlie";
}
console.log(name); // ✅ still works outside the block!

That’s because var has function scope, not block scope.
You can even access it before the declaration, due to hoisting:

console.log(a); // undefined
var a = 10;

And here’s what function scope really means:

function test() {
  if (true) {
    var greeting = "Hi!";
  }
  console.log(greeting); // ✅ "Hi!"
}
test();

console.log(greeting); // ❌ ReferenceError

🧓 Nowadays, we use let and const instead — but it’s still good to know what var does.
You’ll definitely encounter it in older codebases.

🧠 Quick Summary

Keyword	Can Reassign?	Scope	Hoisted?	Use It?
`const`	❌ No	Block	✅ (TDZ)	✅ Default
`let`	✅ Yes	Block	✅ (TDZ)	✅ When value changes
`var`	✅ Yes	Function	✅ (initialized as `undefined`)	🚫 Avoid (legacy)