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JavaScript const

A Variable That Won't Change

JavaScript const

The `const` keyword declares a block-scoped binding that cannot be reassigned. Default to `const` — use `let` only when you actually need to reassign.

4 min read Level 1/5 #const#constants#variables
What you'll learn
  • Declare constants with `const`
  • Understand the difference between a constant binding and a constant value
  • Default to `const` in new code

const declares a binding that cannot be reassigned. Once you attach a value to a const name, that name keeps pointing at the same value for the rest of its scope.

const won't be reassigned script.js 
const TAX_RATE = 0.07;
console.log(TAX_RATE); // 0.07

// TAX_RATE = 0.10; // TypeError: Assignment to constant variable.
▶ Preview: console

Default to const

Modern JavaScript convention: use const everywhere by default, and only use let when you actually need to reassign.

Why? Reading code is easier when you can trust that a name keeps pointing at the same thing. const is a signal: “this won’t change.”

Binding vs Value

A const makes the binding constant — the name keeps pointing at the same value. It does not make the value itself immutable. If the value is an object or array, you can still change what’s inside it.

const objects can still mutate script.js 
const user = { name: "Ada", age: 36 };

user.age = 37;          // ✅ allowed — mutating a property
user.email = "a@b.c";   // ✅ allowed — adding a property

console.log(user);      // { name: 'Ada', age: 37, email: 'a@b.c' }

// user = { name: "Lin" };  // ❌ TypeError — reassigning the binding
▶ Preview: console

To actually freeze an object’s properties, use Object.freeze(). We’ll cover that in the Objects chapter.

Block Scope (Same as let)

Like let, a const only exists inside its block:

const is block-scoped script.js 
{
  const message = "inside";
  console.log(message);
}

// console.log(message); // ReferenceError
▶ Preview: console

Must Initialize

Unlike let, a const must be given a value when you declare it.

const must be initialized script.js 
const pi = 3.14159; // ✅

// const x;          // ❌ SyntaxError: Missing initializer in const declaration
▶ Preview: console

When let Wins

Stick with let only when:

  • You’re counting (let count = 0; count++;).
  • You’re reassigning inside a loop or conditional.
  • You’re swapping or pivoting a value.

Everywhere else, const is your friend.

Up Next

You can now store values. Time to do something with them — operators.

JavaScript Operators →
Reference