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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 Const

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📘 JavaScript const – Declaring Constants in JavaScript

The const keyword in JavaScript is used to declare block-scoped, read-only variables. Once a variable is declared with const, its value cannot be reassigned.

🔹 Syntax:
const pi = 3.14159;
🔹 Key Features of const:

  • 🟡 Must be initialized at declaration

  • 🟡 Cannot be reassigned

  • 🟡 Is block-scoped like let

  • 🟡 Does not allow redeclaration in the same scope

✅ Valid:
const name = "John";
❌ Invalid:
const age;         // ❌ Error: Missing initializer
const age = 30;
age = 35;          // ❌ Error: Assignment to constant variable
🔄 const With Objects and Arrays

const does not make objects/arrays immutable, but it prevents reassignment of the variable itself.

const person = { name: "Alice", age: 25 };
person.age = 30;          // ✅ Allowed
person = { name: "Bob" }; // ❌ Error

const nums = [1, 2, 3];
nums.push(4);             // ✅ Allowed
nums = [4, 5];            // ❌ Error
🔁 Difference Between let and const
Feature let const
Reassignment ✅ Allowed ❌ Not Allowed
Initialization Optional ✅ Required
Scope Block-scoped Block-scoped
Redeclaration ❌ Not allowed ❌ Not allowed

Practice Questions

Q1. How do you declare a constant variable named country with the value "India" using const?

Q2. How do you demonstrate that reassignment of a const variable causes an error?

Q3. How do you declare a constant object car and modify one of its properties (e.g., change color)?

Q4. How do you declare a constant array fruits with three elements and add a fourth element to it?

Q5. How do you show that redeclaring a const variable in the same scope causes an error?

Q6. How do you demonstrate that const is block-scoped using an if block?

Q7. How do you create a constant object user and log one of its properties (like user.name)?

Q8. How do you explain the difference between modifying an object vs. reassigning a const object? Show with code.

Q9. What happens if you declare a const variable without assigning a value? Show with code.

Q10. How do you write a const variable named MAX_SCORE with a numeric value and use it in a condition?

JS Const Quiz

Q1: Which keyword is used to declare a variable that cannot be reassigned?

A. let
B. const
C. static
D. final

Q2: What will happen if you try to reassign a const variable?

A. Nothing happens
B. It will overwrite the value
C. A runtime error is thrown
D. A warning is shown in console

Q3: Which of the following is TRUE about const variables?

A. They must be initialized at declaration
B. They can be redeclared
C. They are not block-scoped
D. Their value can always change

Q4: What is allowed with a const object?

A. Reassign the object
B. Add properties to the object
C. Redeclare the object
D. Reinitialize the object

Q5: Which of the following will cause an error?

A. const x = 5;
B. const x = 5; x = 10;
C. const user = { name: "Ali" };
D. const arr = [1,2,3]; arr.push(4);

Q6: What will this code do? const city; city = "Delhi";

A. Declare and assign value
B. Compile without issue
C. Throw a syntax error
D. Log undefined

Q7: What happens if you modify the contents of an array declared with const?

A. It’s allowed
B. It throws an error
C. It is ignored
D. The variable is deleted

Q8: Which is a valid const declaration?

A. const num;
B. const num =;
C. const num = 10;
D. const = 10;

Q9: Can const be used to declare block-scoped variables?

A. No, only let can
B. Yes, but only inside functions
C. Yes
D. Only in global scope

Q10: What happens when you try to redeclare a const variable in the same block?

A. New value replaces old one
B. Nothing
C. SyntaxError occurs
D. Variable gets updated
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