In JavaScript, `let` is a keyword used to declare variables that are block-scoped, meaning they are only accessible within the block of code where they are defined, such as within a pair of curly braces `{}`. This is different from `var`, which is function-scoped or globally scoped.
Here's a basic example demonstrating the use of `let`:
function example() {
let x = 10;
if (true) {
let x = 20;
console.log(x); // Output: 20
}
console.log(x); // Output: 10
}
example();
In this example:
1. `let x = 10;` declares a variable `x` within the `example` function.
2. Within the `if` block, `let x = 20;` declares a new `x` that is scoped to the `if` block.
3. Inside the `if` block, `console.log(x);` outputs `20`.
4. Outside the `if` block but inside the `example` function, `console.log(x);` outputs `10`, showing that the `x` declared in the `if` block does not affect the `x` declared outside of it.
Key points about `let`:
Block scope:
Variables declared with `let` are limited to the block, statement, or expression where they are used.
No hoisting like `var`:
While variables declared with `var` are hoisted to the top of their function scope, `let` declarations are not initialized until their definition is evaluated.
Temporal Dead Zone (TDZ):
Variables declared with `let` are in a "temporal dead zone" from the start of the block until the declaration is encountered, meaning they cannot be accessed before their declaration.
Using `let` helps prevent common bugs associated with variable scope and provides more predictable and maintainable code.