Variable declaration using const in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of const with object mutation

What is the output of the following code?

Javascript

const obj = { a: 1 };
obj.a = 2;
console.log(obj.a);

A undefined

B 1

C TypeError

D 2

💡 Hint

Remember, const prevents reassignment but not mutation of object properties.

The const keyword prevents reassigning the variable obj, but the object it points to can be changed. So obj.a is updated to 2.

❓ Predict Output

intermediate

Output of const with primitive reassignment

What happens when you run this code?

Javascript

const x = 5;
x = 10;
console.log(x);

A TypeError

B 10

C 5

D ReferenceError

💡 Hint

Can you change the value of a const variable?

Trying to reassign a const variable causes a TypeError because const variables cannot be reassigned.

🧠 Conceptual

advanced

Why use const for arrays?

Which statement about const arrays is true?

A You cannot add or remove elements from a const array.

B You can change elements inside a const array but cannot reassign the array variable.

C You can reassign a const array variable to a new array.

D Const arrays are immutable and cannot be changed in any way.

💡 Hint

Think about what const protects: the variable binding or the content.

Const protects the variable binding, so you cannot reassign the array variable, but you can change the contents of the array.

❓ Predict Output

advanced

Output of const in block scope

What will this code print?

Javascript

const a = 1;
{
  const a = 2;
  console.log(a);
}
console.log(a);

A 2 1

B 2 2

C 1 2

D 1 1

💡 Hint

Remember const variables are block scoped.

Inside the block, a new const a = 2 is declared, so console.log(a) inside prints 2. Outside the block, the original a = 1 remains, so console.log(a) prints 1.

❓ Predict Output

expert

Output of const with destructuring and reassignment

What is the output of this code?

Javascript

const [x, y] = [1, 2];
x = 3;
console.log(x, y);

A 3 2

B 1 2

C TypeError

D ReferenceError

💡 Hint

Can you reassign a const variable after destructuring?

x is declared as a const variable via destructuring. Trying to reassign it causes a TypeError.

Practice

1. What does the const keyword do in JavaScript?

easy

A. Declares a variable that cannot be reassigned

B. Declares a variable that can be reassigned anytime

C. Declares a function

D. Declares a variable only inside loops

Solution

1. Step 1: Understand const behavior

   The const keyword creates a variable that cannot be reassigned after its initial value is set.

2. Step 2: Compare options

   Only Declares a variable that cannot be reassigned correctly states that const variables cannot be reassigned.

3. Final Answer:

   Declares a variable that cannot be reassigned -> Option A

4. Quick Check:

   const means no reassignment [OK]

Hint: Remember: const means constant value, no changes allowed [OK]

Common Mistakes:

• Thinking const variables can be reassigned
• Confusing const with let or var
• Assuming const declares functions

2. Which of the following is the correct syntax to declare a constant variable named pi with value 3.14?

easy

A. constant pi = 3.14;

B. const pi := 3.14;

C. const pi = 3.14;

D. let const pi = 3.14;

Solution

1. Step 1: Recall correct const syntax

   In JavaScript, the correct syntax to declare a constant is const variableName = value;.

2. Step 2: Check each option

   Only const pi = 3.14; matches the correct syntax. The other options use invalid keywords or symbols.

3. Final Answer:

   const pi = 3.14; -> Option C

4. Quick Check:

   Correct const syntax uses = and no extra keywords [OK]

Hint: Use const name = value; with equal sign [OK]

Common Mistakes:

• Using := instead of =
• Writing 'constant' instead of 'const'
• Combining let and const keywords

3. What will be the output of this code?

const greeting = 'Hello';
greeting = 'Hi';
console.log(greeting);

medium

A. undefined

B. Hi

C. Hello

D. TypeError

Solution

1. Step 1: Understand reassignment of const variable

   The variable greeting is declared with const, so it cannot be reassigned after initialization.

2. Step 2: Analyze the reassignment line

   Trying to assign 'Hi' to greeting causes a TypeError at runtime.

3. Final Answer:

   TypeError -> Option D

4. Quick Check:

   Reassigning const variable throws TypeError [OK]

Hint: const variables cannot be reassigned; error occurs if tried [OK]

Common Mistakes:

• Assuming const variables can be changed
• Expecting console.log to print new value
• Confusing TypeError with SyntaxError

4. Find the error in this code snippet:

const number;
number = 10;
console.log(number);

medium

A. Missing value when declaring const variable

B. Cannot use console.log with const

C. Variable name 'number' is reserved

D. No error, code runs fine

Solution

1. Step 1: Check const declaration rules

   When declaring a const variable, it must be initialized with a value immediately.

2. Step 2: Identify the problem in code

   The code declares const number; without assigning a value, which causes a SyntaxError.

3. Final Answer:

   Missing value when declaring const variable -> Option A

4. Quick Check:

   const must be initialized on declaration [OK]

Hint: Always assign a value when declaring const [OK]

Common Mistakes:

• Declaring const without initialization
• Thinking console.log causes error
• Assuming variable names like 'number' are invalid

5. You want to store a list of colors that should never change in your program. Which is the best way to declare it using const and why?

const colors = ['red', 'green', 'blue'];
colors.push('yellow');
console.log(colors);

hard

A. Use const and avoid changing array contents; push will cause error

B. Use const for the array; you can change contents but not reassign the variable

C. Use let instead of const to allow adding colors

D. Declare colors as a string instead of array

Solution

1. Step 1: Understand const with objects and arrays

   Using const means the variable cannot be reassigned, but the contents of objects or arrays can be changed.

2. Step 2: Analyze the code behavior

   The array colors is declared with const, but push modifies its contents without reassigning the variable, so it works fine.

3. Final Answer:

   Use const for the array; you can change contents but not reassign the variable -> Option B

4. Quick Check:

   const protects variable binding, not object contents [OK]

Hint: const protects variable name, not array contents [OK]

Common Mistakes:

• Thinking const makes array immutable
• Using let when const is better for fixed references
• Confusing reassignment with content modification