Var, let, const: clean code starts here

When users experience slow pages, strange bugs, or things that “just don’t work right,” it's often rooted in tiny decisions — like how you declare your variables.

Yes, even var, let, and const matter.

Let’s talk about what they do, and more importantly, why choosing the right one leads to better apps.

🕹️ What’s the difference?

• TDZ = Temporal Dead Zone (you can’t use the variable before it’s declared)

⚠️ Why var is a problem

Example:

if (true) {
  var message = 'Hello'
}
console.log(message) // "Hello"

It leaks out of the block, because var ignores {}.

This kind of behavior can cause:

• Conflicts in large codebases
• Undesired global variables
• Bugs that are hard to track in async or dynamic UIs

For the user, that might look like:

• A button stops working
• Data disappears unexpectedly
• Forms behaving inconsistently

All from an outdated keyword.

✅ Why let and const are safer

They respect block scope:

if (true) {
  let name = 'Camila'
  const age = 30
}
console.log(name) // ❌ ReferenceError

So which one should you use?

👉 Use const by default.

It signals intent: “I won’t reassign this.”

const API_URL = '/api/posts'

👉 Use let when the value needs to change.

let counter = 0
counter++

This makes your code more predictable, which means:

• Fewer bugs
• Easier to read
• Better behavior under pressure (e.g. user clicking fast, bad network)

🧠 Real-world example: a cart button bug

// Bad
var quantity = 1
button.addEventListener('click', () => {
  console.log(quantity)
})
quantity = 5

🛑 User clicks, sees wrong quantity in console or UI.

// Good
const quantity = 1
button.addEventListener('click', () => {
  console.log(quantity)
})

🟢 Now it's locked — no weirdness. What the dev sees is what the user gets.

🔄 Performance?

var, let, and const perform about the same.

The difference isn’t speed — it’s clarity and correctness, which leads to better UX.

📎 Conclusions

Using let and const isn’t about being modern — it’s about writing code users can trust.

Every time you prevent a scope leak or an accidental reassignment, you're building an app that:

• Behaves consistently
• Handles edge cases better
• Makes users feel in control

Use const by default. Reach for let when needed.

And leave var in 2009 where it belongs.