The let Keyword

let declares a block-scoped variable that can be reassigned.

Syntax

let count = 0;
count = count + 1;

let declares a variable whose value can change later. It is block-scoped, meaning it only exists inside the { } block where it is declared.

You can change a let variable as many times as you like — but you declare it with let only once.

Example

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Track a loop counter variable that should not be accessible outside the for-block.
Store form input state that may change as the user edits fields before submission.
Switch a UI mode variable between "dark" and "light" when the user toggles the theme.

Shows that a let variable declared in a for-loop header is scoped to that block only.

for (let i = 0; i < 3; i++) {
  console.log(i);
}
// console.log(i); // ReferenceError – i is not defined here

Demonstrates that let allows the variable to be reassigned, unlike const.

let theme = "light";
theme = "dark";
console.log(theme); // dark

Illustrates the temporal dead zone: accessing a let variable before its declaration throws a ReferenceError.

// console.log(count); // ReferenceError: cannot access before init
let count = 10;
console.log(count); // 10