Nested objects in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Accessing nested object properties

What is the output of the following code?

Javascript

const data = {
  user: {
    id: 101,
    profile: {
      name: "Alice",
      age: 30
    }
  }
};

console.log(data.user.profile.name);

A undefined

B "Alice"

C null

D TypeError

💡 Hint

Look at how to access properties inside nested objects using dot notation.

The code accesses the 'name' property inside the 'profile' object, which is inside 'user'. The value is "Alice".

❓ Predict Output

intermediate

Modifying nested object values

What will be the output after running this code?

Javascript

const obj = {
  a: {
    b: {
      c: 5
    }
  }
};

obj.a.b.c = 10;
console.log(obj.a.b.c);

A 10

B undefined

C 5

D TypeError

💡 Hint

Think about how assignment works on nested objects.

The code changes the value of 'c' from 5 to 10, so logging it prints 10.

❓ Predict Output

advanced

Accessing nested object with optional chaining

What is the output of this code snippet?

Javascript

const user = {
  name: "Bob",
  address: {
    city: "New York"
  }
};

console.log(user.address?.street);

A null

B TypeError

C "" (empty string)

D undefined

💡 Hint

Optional chaining returns undefined if the property does not exist.

The 'street' property does not exist inside 'address', so optional chaining returns undefined without error.

❓ Predict Output

advanced

Deeply nested object property count

How many properties does the object 'data' have at the deepest level?

Javascript

const data = {
  level1: {
    level2: {
      level3: {
        a: 1,
        b: 2,
        c: 3
      }
    }
  }
};

console.log(Object.keys(data.level1.level2.level3).length);

A 0

B 1

C 3

D TypeError

💡 Hint

Count the keys at the deepest nested object.

The deepest object has keys 'a', 'b', and 'c', so the count is 3.

❓ Predict Output

expert

Merging nested objects with spread operator

What is the output of this code?

Javascript

const obj1 = { a: 1, b: { c: 2 } };
const obj2 = { b: { d: 3 }, e: 4 };

const merged = { ...obj1, ...obj2 };
console.log(merged.b.c);

A undefined

B 3

C 2

D TypeError

💡 Hint

Spread operator does a shallow merge, so nested objects get replaced, not merged.

The 'b' property from obj2 replaces the one from obj1, so merged.b only has 'd', no 'c'. Accessing merged.b.c returns undefined.