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.

Practice

1. What is a nested object in JavaScript?

easy

A. An array inside an object

B. An object inside another object

C. A function inside an object

D. A string inside an object

Solution

1. Step 1: Understand object structure

   An object can hold many types of values, including other objects.

2. Step 2: Define nested object

   A nested object means one object is stored as a value inside another object.

3. Final Answer:

   An object inside another object -> Option B

4. Quick Check:

   Nested object = object inside object [OK]

Hint: Nested means inside another object, like boxes in boxes [OK]

Common Mistakes:

• Thinking nested means array inside object
• Confusing functions with objects
• Assuming nested means string inside object

2. Which of the following is the correct syntax to access the value 42 in this nested object?

const data = { info: { age: 42 } };

easy

A. data.info.age

B. data[info][age]

C. data['info'.'age']

D. data.info['age']()

Solution

1. Step 1: Access nested keys with dot notation

   Use dot notation to access nested object properties: data.info.age.

2. Step 2: Check syntax correctness

   data[info][age] is wrong because keys must be strings or variables; data['info'.'age'] is invalid syntax; data.info['age']() tries to call a value as function.

3. Final Answer:

   data.info.age -> Option A

4. Quick Check:

   Dot notation for nested keys = data.info.age [OK]

Hint: Use dot notation for nested keys like data.key1.key2 [OK]

Common Mistakes:

• Using brackets without quotes for keys
• Trying to call a property as a function
• Incorrect string concatenation in keys

3. What will be the output of this code?

const user = { name: 'Alice', contact: { email: '[e-mail]', phone: '1234' } };
console.log(user.contact.email);

medium

A. '[e-mail]'

B. '1234'

C. 'contact'

D. undefined

Solution

1. Step 1: Identify nested property access

   user.contact.email accesses the 'email' property inside the 'contact' object.

2. Step 2: Check the value of user.contact.email

   The value is the string '[e-mail]'.

3. Final Answer:

   '[e-mail]' -> Option A

4. Quick Check:

   Access nested email = '[e-mail]' [OK]

Hint: Follow the chain: object.property.property to get nested value [OK]

Common Mistakes:

• Accessing wrong property like user.email
• Using wrong quotes causing syntax error
• Expecting phone instead of email

4. Find the error in this code snippet:

const book = { title: 'JS Guide', author: { firstName: 'John', lastName: 'Doe' } };
console.log(book.author.firstName());

medium

A. Incorrect object declaration

B. Missing semicolon after object

C. firstName is not a function

D. author property does not exist

Solution

1. Step 1: Check property type

   book.author.firstName is a string, not a function.

2. Step 2: Identify the error in calling it

   Using parentheses () tries to call it as a function, causing a TypeError.

3. Final Answer:

   firstName is not a function -> Option C

4. Quick Check:

   Calling string as function causes error [OK]

Hint: Don't add () after properties unless they are functions [OK]

Common Mistakes:

• Calling string property as function
• Ignoring error message
• Assuming missing semicolon causes error

5. Given this nested object:

const company = {
  name: 'TechCorp',
  departments: {
    sales: { employees: 10 },
    development: { employees: 25 },
    hr: { employees: 5 }
  }
};

// How to get total employees in all departments?

Which code correctly calculates total employees?

hard

A. company.departments.sales + company.departments.development + company.departments.hr

B. company.departments.employees.sales + company.departments.employees.development + company.departments.employees.hr

C. company.departments['employees'].sales + company.departments['employees'].development + company.departments['employees'].hr

D. company.departments.sales.employees + company.departments.development.employees + company.departments.hr.employees

Solution

1. Step 1: Understand object structure

   Each department is an object inside company.departments, with an employees property holding a number.

2. Step 2: Access employees count correctly

   Access employees by chaining: company.departments.sales.employees, etc., then add them.

3. Step 3: Check incorrect options

   Adding the department objects directly (like company.departments.sales) leads to summing objects instead of numbers; accessing employees as a property of departments fails because it doesn't exist on departments--each department has its own.

4. Final Answer:

   company.departments.sales.employees + company.departments.development.employees + company.departments.hr.employees -> Option D

5. Quick Check:

   Sum employees by chaining keys correctly [OK]

Hint: Add numbers by chaining keys: obj.key1.key2 + obj.key3.key4 [OK]

Common Mistakes:

• Adding objects instead of numbers
• Wrong property order
• Using wrong key names