JavaScript — Let

let declares a changeable variable

let score = 0;
score = 100;        // reassignment — fine
score = score + 1;  // 101

Block scope — the big upgrade

Variables born inside { } stay inside:

{
    let temp = "secret";
}
console.log(temp);   // ReferenceError — never left the block

if (true) {
    let x = 1;
    console.log(x);   // 1 ✓ inside is fine
}
console.log(x);       // Error outside

Loops get a fresh copy each iteration — the classic var bug simply can't happen:

for (let i = 0; i < 3; i++) {
    setTimeout(() => console.log(i));   // 0, 1, 2 ✓
}

Rules of engagement

let user;              // declared, value undefined until assigned
let a = 1, b = 2;      // multiple in one line (works, less readable)

user = "Ada";          // second declaration would ERROR:
// let user = "Bo";    // SyntaxError: already declared

Re-declaring errors are a feature — typos can't silently create new variables.

let vs var vs const

	let	const	var
Reassign?	Yes	Never	Yes
Scope	Block	Block	Function
Use today	When it changes	Default	Avoid

Decision flow: start with const; switch to let only when reassignment proves necessary; never reach for var again.

Common mistakes

Redeclaring in the same scope — SyntaxError, not a warning.

Using before declaring — ReferenceError ("temporal dead zone"), unlike var's silent undefined.

Mini Practice

1. Reassign a let three times; log between hops
2. Prove block scope: declare inside an if, read outside, catch the error
3. Try redeclaring; read the exact error text
4. Convert a var loop to let and verify timeouts print 0-1-2

Next: const →