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.