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JavaScript Constants: 7 Powerful Ways to Use const Effectively

JavaScript Constants

What Are JavaScript Constants

JavaScript Constants are variables declared using the const keyword. These values cannot be reassigned once they are created. In simple words:
const = permanent value in your code.

Example:

const PI = 3.14;

Once assigned, you cannot do this:

PI = 5; // ❌ Error

The value remains fixed.

Why JavaScript Introduced const

Before ES6, JavaScript only had var. The problem was:

  • var could be changed anytime
  • var was function-scoped, causing many bugs
  • var allowed accidental reassignments

To make JavaScript safer and more predictable, ES6 introduced:

  • let
  • const

Among these, JavaScript Constants (const) are the safest because the value cannot be changed.

How JavaScript Constants Work

When you declare a variable with const:

  1. You must assign a value immediately
  2. You cannot reassign the value
  3. It is block scoped, not function scoped
  4. Objects and arrays declared with const can change inside, but not be reassigned

Example:

const user = { name: "Ram" };
user.name = "Sita"; // ✔ Allowed
user = {}; // ❌ Not allowed

When You Should Use const

Use JavaScript Constants when:

  • Value should not change
  • You are creating configuration data
  • You want safer code
  • You want predictable behavior
  • The variable is used many times but never needs reassignment

Examples:

const API_URL = "https://api.example.com";
const TAX_RATE = 0.18;
const MAX_USERS = 100;

When You Should Not Use const

Avoid using const when:

  • You KNOW the value will change
  • You are using counters
  • You are using loops
  • You are updating values frequently

Example:

let count = 0; // NOT const
count++;

Best Practices for JavaScript Constants

To use JavaScript Constants effectively:

  • Use uppercase names for global constants (e.g., API_URL)
  • Use const for default values
  • Prefer const over let unless needed
  • Use descriptive names
  • Group constants together

Example:

const BASE_URL = "https://example.com";
const DEFAULT_LANGUAGE = "en";
const VERSION = "1.0.0";

Common Mistakes Beginners Make

Here are some mistakes when using JavaScript Constants:

Not assigning value immediately

const x; // ❌ Error

Trying to reassign a constant

 

const age = 20;
age = 21; // ❌ Not allowed

Thinking arrays/objects are fully frozen

They are not.

const arr = [1, 2];
arr.push(3); // ✔ Allowed
arr = [4]; // ❌ Not allowed

Practical Examples of Using const

 

Using const for configuration

const SITE_NAME = "Byte Summit";
const SUPPORT_EMAIL = "help@bytesummit.com";

Using const with functions

const greet = () => "Hello User";

Using const with arrays

const fruits = ["apple", "banana"];
fruits.push("mango"); // ✔ Allowed

Using const in real-world apps

const BASE_API = "https://myapi.com";
const LOGIN_ROUTE = "/login";
const REGISTER_ROUTE = "/register";

External Links (DoFollow)

Internal Link Suggestions

Add links to your other JS articles, for example:

Why Professionals Prefer JavaScript Constants

Professional JavaScript developers follow a simple rule:

Use const by default, use let only when needed.

Reasons:

  • Cleaner code
  • Fewer bugs
  • Good for teamwork
  • Makes code predictable
  • Works well with modern frameworks

Most coding standards by companies like Google, Airbnb, Meta also prefer const.

Conclusion

JavaScript Constants are an essential part of writing clean, predictable, and bug-free code. The const keyword is one of the best features introduced in ES6, making modern JavaScript easier and safer to write. Whether you are building a simple script, a website, or a full web application, using const helps keep your values secure, your logic clean, and your code professional.

FAQs

What are JavaScript Constants?

JavaScript Constants are variables declared with const whose values cannot be reassigned.

Can arrays declared with const be changed?

Yes, you can modify the contents but cannot reassign the whole array.

Is const faster than let or var?

Yes, const helps JavaScript engines optimize your code better.

Should I use const by default?

Yes, modern JavaScript recommends using const unless you need reassignment.

Can I redeclare a const variable?

No, redeclaring a const variable will throw an error.

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