Mastering JavaScript's `let` Keyword: A Comprehensive Guide

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Introduction to JavaScript Let

In JavaScript, `let` is a keyword that is used to declare block-scoped variables. Unlike the `var` keyword, which is function-scoped, variables declared with `let` are limited in scope to the block, statement, or expression in which they are defined.

Block Scope:

The variables which are declared inside the { } block are known as block-scoped variables. variables declared by the var keyword cannot be block-scoped.

Example: In this example, the variable `num` is declared using the `let` keyword within the statement block. This variable is only accessible within the block, so trying to access it outside of the block will result in a reference error.

{
 
let num=10;
 
// calling the function inside block
 
console.log(num)  //10
 
}
 
// calling the function outside block throws a Error
 
console.log(num)  //Uncaught ReferenceError: num is not defined

Global Scope:

A global scope variable is a variable declared in the main body of the source code, outside all functions.

Example: In this example, the num variable is a globally scoped variable and it can be accessed from anywhere in the program.

let num=10;
 
console.log(num);  //10
 
function fun(){
 
console.log(num);  //10
 
}
 
fun(); // calling the function

Function Scope: 

A function scope variable is a variable declared inside a function and cannot be accessed outside the function.

Example: In this example, the num variable is declared inside the function and cannot be accessed outside the function.

function fun(){
 
let num=10;
 
console.log(num);  //10
 
}
 
fun(); // calling the function
 
console.log(num);  //ReferenceError: num is not defined

Redeclaring Variables in different blocks: 

The variables declared using let can be redeclared inside other blocks.

Example: In this example, variable x is redeclared inside other blocks.

let x=77;
 
{
 
let x=23;
 
console.log(x);  //23
 
}
 
console.log(x);  //77

Redeclaring Variables in the same blocks: 

We cannot redeclare variables using the let keyword inside the same blocks. It will throw an error.

Example: In this example, variable x is redeclared inside same blocks.

let x=77;
 
{
 
let x=23; // legal
 
console.log(x);
 
}
 
let x=67;// illegal
 
console.log(x); //Uncaught SyntaxError: Identifier 'x' has already been declared

Reassigned Variables in the same blocks: 

In this example, the variable `count` is declared with the `let` keyword and assigned a value of 5. Later, the value of `count` is reassigned to 10. This flexibility allows you to easily update the value of `let` variables as needed.

let count = 5;
 
console.log(count); // 5
 
count = 10;
 
console.log(count); // 10

Does not support Hoisting:

The behavior of moving the declarations on top of the script is known as hoisting.

Unlike variables declared with `var`, which are hoisted to the top of their scope, variables declared with `let` are not hoisted. This means that you need to declare them before you can use them:

console.log(count); // This will throw an error
 
let count = 5;

In this example, trying to access the variable `count` before it is declared will result in an error. It's important to note that although `let` variables are not hoisted, they are still subject to temporal dead zones (TDZ). This means that if you try to access a `let` variable before it is declared in the same block, you will get a reference error.

Key Takeways:

  • `let` variables are block-scoped and not accessible outside of the block in which they are declared.

  • `let` variables can be reassigned with a new value.

  • `let` variables are not hoisted, so they need to be declared before they can be used.

  • `let` variables are also subject to temporal dead zones (TDZ) if they are accessed before they are declared in the same block.

With a good understanding of the `let` keyword, you can effectively control the scope and reassignment of variables in your JavaScript code.

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