JavaScript Let

Declare with let

let creates a name in the current block. You may give it a value on the same line, or assign later. After that, you can store a new value in the same name.

let city = "Paris";
console.log(city);
city = "Lisbon";
console.log(city);

The name city stays. The string inside it changes. That is reassignment.

Reassign, do not redeclare

Write city = "Lisbon" to change the value. Do not write let city again in the same block. A second declaration with the same name is a syntax error.

let n = 1;
n = 2;
console.log(n);
// let n = 3;  // SyntaxError: already declared

One let per name per block. After that, only assignment.

Block scope

A block is the code between braces: the body of an if, a loop, or a pair of braces you write yourself. A let declared inside a block is not visible outside that block.

let score = 12;
if (score >= 10) {
  let message = "pass";
  console.log(message);
}
// console.log(message); // ReferenceError: message is not defined
console.log(score);

score lives in the outer block, so the last log works. message lives only inside the if. That is what “block-scoped” means.

Inner names can reuse a word

An inner block may declare its own let with a name that already exists outside. Inside the block, that inner name wins. Outside, the outer name is unchanged. This is legal. It is also easy to misread. Prefer different names.

let label = "outer";
{
  let label = "inner";
  console.log(label);
}
console.log(label);

The console prints inner, then outer. Two boxes, same word, different blocks.

Use the name after the declaration

You cannot read a let on the lines above its declaration in the same block. The name exists on paper, but it is not ready. That gap is called the temporal dead zone. Declare first, then use.

let vs var

var ignores block braces. A var inside an if still leaks to the function around it. let does not leak. That is the main reason this tutorial useslet.

When to use let

Use let for a counter, a running total, or any name you will assign more than once. If you never reassign, use const instead. That is the next chapter.