JavaScript Block Scope

JavaScript block scope means let and const variables only exist inside the braces where they were declared.

A block is any pair of braces: an if, a loop, or a plain pair on its own.

var ignores blocks entirely, which is the main reason it causes confusion.

Example

if (true) {
  let message = "inside";
  console.log(message);
}

console.log(typeof message);

The variable exists inside the block.

Outside, it is undefined, because it does not exist there at all.