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JavaScript

JS Basics

JS Variables & Operators

JS Data Types & Conversion

JS Numbers & Math

JS Strings

JS Dates

JS Arrays

JS Control Flow

JS Loops & Iteration

JS Functions

JS Objects

JS Classes & Modules

JS Async Programming

JS Advanced

JS HTML DOM

JS BOM (Browser Object Model)

JS Web APIs

JS AJAX

JS JSON

JS Graphics & Charts

JS Let

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📘 JavaScript let – Block Scoped Variable Declaration

The let keyword was introduced in ES6 (2015) and is used to declare variables that are block-scoped, meaning they are only accessible within the block {} in which they are defined.

🔹 Syntax:
let x = 5;
let name = "John";

🔹 Why Use let?

  • Block scope (unlike var which is function-scoped)

  • Prevents accidental redeclarations in the same scope

  • Safer and more predictable for loops, conditions, and modern JS practices

🧱 Block Scope Example:

{
  let x = 10;
  console.log(x); // ✅ 10
}
console.log(x); // ❌ ReferenceError: x is not defined

🚫 Cannot Redeclare let in the Same Scope

let name = "Alice";
let name = "Bob"; // ❌ Error: Identifier 'name' has already been declared

🔄 But Can Reassign let Variables

let city = "Delhi";
city = "Mumbai"; // ✅ This is allowed

🔄 Difference Between var and let

Feature var let
Scope Function-scoped Block-scoped
Redeclaration Allowed Not allowed
Hoisting Hoisted (value: undefined) Hoisted (but not initialized)
Reassignment Allowed Allowed

Practice Questions

Q1. How do you declare a variable named age using the let keyword and assign it the value 30?

Q2. How do you update a variable city declared with let to change its value from "Delhi" to "Mumbai"?

Q3. How do you demonstrate block scope by declaring a let variable inside a block and trying to access it outside the block?

Q4. How do you show that a variable declared with let cannot be redeclared in the same scope?

Q5. How do you use a let variable in a for loop to iterate from 1 to 5 and print each number?

Q6. How do you write a JavaScript code that declares the same variable name with let inside and outside a block to show they are treated separately?

Q7. How do you declare a variable x with let, assign it 10, then reassign it to 20 and log the result?

Q8. How can you demonstrate that let declarations are hoisted but not initialized?

Q9. How do you write a function with a let variable that does not affect the global variable with the same name?

Q10. How do you write a condition using if block where a let variable is declared inside and used only within that block?

JS Let Quiz

Q1: What does the let keyword do in JavaScript?

A. Declares a constant variable
B. Declares a block-scoped variable
C. Declares a function
D. Creates a loop

Q2: Which of the following is true about let declarations?

A. They are function-scoped
B. They allow redeclaration
C. They are block-scoped
D. They are not hoisted

Q3: What happens if you try to access a let variable before it is declared?

A. It returns undefined
B. It throws a ReferenceError
C. It returns null
D. It auto-declares the variable

Q4: Which keyword prevents redeclaring the same variable in the same scope?

A. var
B. let
C. function
D. define

Q5: Which variable declaration is block-scoped?

A. var
B. let
C. Both a and b
D. None of the above

Q6: What happens if you redeclare a let variable in the same block?

A. No error, value is overwritten
B. SyntaxError is thrown
C. Value becomes null
D. The program skips the line

Q7: Can you reassign a value to a let variable?

A. No
B. Only once
C. Yes
D. Only inside a loop

Q8: Which of the following best describes the scoping of let?

A. Global
B. Local
C. Block
D. Script

Q9: Which of the following is NOT allowed when using let?

A. Declaring inside a block
B. Reassigning a value
C. Redeclaring in the same scope
D. Using in a loop
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