In JavaScript, the let keyword is used to declare a block-scoped variable. It was introduced in ECMAScript 6 (ES6) and is the preferred way to declare variables in modern JavaScript.

ES6 (ECMAScript 2015) is a major update to JavaScript that adds new syntax and features to the language. It was officially released in June 2015.

The syntax to declare a variable using the let keyword is:

let variableName = value;

Here, variableName is the name of the variable and value is the value assigned to the variable.

Let's see an example to understand how to use the let keyword in JavaScript:

let name = "John";
console.log(name); // Output: John

In the above example, we have declared a variable name using the let keyword and assigned it a value "John". We then printed the value of the variable using console.log().

The let keyword is block-scoped, which means that the variable declared using let is only available within the block in which it is defined.

{
  let x = 10;
  console.log(x); // Output: 10
}

console.log(x); // Uncaught ReferenceError: x is not defined

In the above example, the variable x is only available within the block in which it is defined. If we try to access it outside the block, it will result in a ReferenceError.

The let keyword does not allow redeclaration of the same variable within the same block scope.

let x = 10;
let x = 20; // Uncaught SyntaxError: Identifier 'x' has already been declared

In the above example, we are trying to redeclare the variable x within the same block scope, which results in a SyntaxError.

Variables declared using the let keyword are not hoisted to the top of their block. This means that you cannot access the variable before it is declared.

console.log(x); // Uncaught ReferenceError: x is not defined
let x = 10;

In the above example, we are trying to access the variable x before it is declared, which results in a ReferenceError.

The let keyword differs from the var keyword in terms of block scope, redeclaration, and hoisting.

Variables declared using the var keyword are function-scoped or globally-scoped, whereas variables declared using the let keyword are block-scoped.

// var
function varExample() {
  if (true) {
    var x = 10;
  }
  console.log(x); // Output: 10
}

varExample();

// let
function letExample() {
  if (true) {
    let x = 10;
  }
  console.log(x); // Uncaught ReferenceError: x is not defined
}

letExample();

In the above example, the variable x declared using the var keyword is available outside the block in which it is defined, whereas the variable x declared using the let keyword is not available outside the block.

:::info A block is a set of statements enclosed in curly braces {}. :::

In this article, we learned about the let keyword in JavaScript and how to use it to declare block-scoped variables. We also saw how the let keyword differs from the var keyword in terms of block scope, redeclaration, and hoisting.