JavaScript — Const

const = binding that can't be reassigned

const BIRTHDAY = "1990-05-01";
BIRTHDAY = "2000-01-01";   // TypeError: Assignment to constant variable

Declare and initialize in one step — there's no "later":

const PI;          // SyntaxError: missing initializer
PI = 3.14;

The modern default

Style guides agree: declare everything const, downgrade to let only when reassignment is truly needed. Why:

1. Readers instantly know this name never flips
2. Accidental reassignments become loud errors
3. Intent becomes documentation

Objects: constant binding ≠ frozen value

const locks the name→value link, not the value's internals:

const user = { name: "Ada" };
user.name = "Grace";      // ✓ mutating properties is fine
user.age = 36;            // ✓ adding works too

user = {};                // ✗ TypeError — rebinding the NAME fails

Same story with arrays:

const colors = ["red"];
colors.push("blue");      // ✓ array grows
colors = [];              // ✗ error

Need true immutability? Ask explicitly:

Object.freeze(user);       // shallow-frozen object

Naming conventions

const MAX_USERS = 100;        // true constants: UPPER_SNAKE_CASE
const API_URL = "/api/v1";

const currentUser = getSession();  // regular values: normal camelCase

UPPERCASE signals "fixed configuration"; everyday consts stay camelCase.

Scope matches let

Block-scoped, no hoisting access before declaration line:

if (true) {
    const local = "here only";
}
// console.log(local) → ReferenceError

Common mistakes

Initializing late — must assign at declaration. Expecting const arrays/objects to be immutable — freeze explicitly if needed.

Mini Practice

1. Trigger the assignment TypeError; read it twice
2. Mutate a const object's property successfully — internalize the distinction
3. Refactor five lets that never change into consts
4. Try declaring without initializing; note the syntax error
5. Freeze an object; attempt a mutation; observe silent failure (strict mode throws)

Next: arithmetic →