Using the const Keyword in JavaScript

Constant Variables (But Not as Constant as You Think 👀)

When you start writing JavaScript, you quickly meet a trio:

• var → almost retired, but still hanging around
• let → modern, logical, reliable
• const → once it gives its word, it never backs down 😎

In this article, the spotlight is on const.
But here’s a small spoiler: const does not always mean “absolutely unchangeable.”
Yes, JavaScript is once again inviting us to a little mental workout.

What Is const? What Does It Do? What Doesn’t It Do?

const is used to define variables that cannot be reassigned.

const siteName = "CodeDünyası";

With this line, you’re basically telling JavaScript:

“Hey buddy, this variable is called siteName.
This is its value.
Don’t try to overwrite it.”

Now let’s do this:

siteName = "AnotherSite";

JavaScript reacts immediately:

❌ TypeError: Assignment to constant variable

Which, translated from JS language, means:

“I’m not allowing this. We had an agreement.”

Summary: const = no reassignment ❌

The Golden Rule of Using const 🏆

A const must receive a value at the moment it is declared.

Wrong usage (JavaScript gets angry):

const age;
age = 25;

Correct usage:

const age = 25;

JavaScript is very clear here:

• “I’ll assign it later” ❌
• “Let me think about it” ❌
• “I’m not sure yet” ❌

If you declare it, you assign it. End of story.

Why Does Everyone Use const?

Because const:

• Communicates intent in your code
• Eliminates the risk of accidental reassignment
• Tells your teammates (and your future self): “This value won’t change, don’t waste time touching it.”

In modern JavaScript, there’s an unwritten but widely accepted rule:

Use const first, switch to let only if necessary

So your default reflex should be:

const total = 100;

If it really needs to change:

let total = 100;
total = 120;

Plot Twist 🎬 const + Objects

Now we’re getting to the fun part.

const user = {
  name: "Ahmet",
  age: 30
};

This variable is declared with const. So what happens next?

1️⃣ Modifying the object’s contents

user.age = 31;

✅ WORKS

Why?

• const locks the reference
• The inside of the object is a free playground 🎈

JavaScript is basically saying:

“You can keep using the same object, just feel free to rearrange what’s inside.”

2️⃣ Replacing the entire object

user = { name: "Mehmet", age: 25 };

❌ ERROR

Because this is a brand-new reference, and const does not allow that.

Same Story: const + Arrays 🍿

const numbers = [1, 2, 3];

Let’s modify the array:

numbers.push(4);

✅ Works

numbers[0] = 99;

✅ Works

But if you do this:

numbers = [5, 6, 7];

❌ Error

In short:

• The content can change
• The reference cannot

If You Want const to Be Really Constant 🧊

If you want to prevent even internal changes:

const settings = Object.freeze({
  theme: "dark",
  language: "tr"
});

settings.theme = "light";

❌ No change (throws an error in strict mode)

💡 Tip: Object.freeze is shallow. Nested objects need extra precautions.

Where Is const Used in Real Life?

🔗 API URLs

const API_URL = "https://api.example.com";

🧮 Functions

const calculateTotal = (price, tax) => price + tax;

🖱️ DOM elements

const submitButton = document.querySelector("#submitBtn");

⚙️ Configuration values

const MAX_RETRY_COUNT = 3;

If these values aren’t supposed to change, const is your best friend ❤️

Small Mistakes, Big Lessons 🚧

❌ Using let everywhere

Unnecessary risk

❌ Misunderstanding const

“Why did this change?” panic

✅ Conscious use of const

Clean, safe, readable code

Burn It Into Your Brain 🧠 (Ultra Summary)

• const prevents reassignment
• It must be initialized immediately
• Object and array contents can change
• References cannot change
• It’s the default choice in modern JavaScript

Closing 🎤

In JavaScript, const:

• Communicates intent
• Reduces bugs
• Protects you from yourself 😄

If you still reflexively write let everywhere…
somewhere out there, a const is quietly feeling sad.