Nested objects in Javascript - Step-by-Step Execution

Concept Flow - Nested objects

Start

↓

Create outer object

↓

Add inner object as property

↓

Access or modify inner object property

↓

Use nested values

↓

End

This flow shows how we create an object that contains another object inside it, then access or change the inner object's properties.

Execution Sample

Javascript

const person = {
  name: "Alice",
  address: {
    city: "Wonderland",
    zip: 12345
  }
};
console.log(person.address.city);

This code creates a person object with a nested address object, then prints the city inside the address.

Execution Table

Step	Action	Object State	Output
1	Create person object with name and address properties	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}
2	Access person.address.city	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}	"Wonderland"
3	Print city value	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}	Wonderland
4	End of code execution	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}

💡 Code ends after printing the nested city value.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
person	undefined	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}	{"name":"Alice","address":{"city":"Wonderland","zip":12345}}
person.address.city	undefined	"Wonderland"	"Wonderland"	"Wonderland"

Key Moments - 2 Insights

Why do we write person.address.city instead of just person.city?

Because city is inside the nested address object, not directly inside person. See execution_table step 2 where we access person.address.city.

Can we change the city value directly inside the nested object?

Yes, by assigning a new value to person.address.city. The nested object is just like any other object property. This is implied by the structure in execution_table step 1.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the value of person.address.city?

A 12345

B "Alice"

C "Wonderland"

D undefined

💡 Hint

Check the Output column at step 2 in the execution_table.

At which step does the code print the city name?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the step where Output shows the printed value.

If we add a new property person.address.street, where would it appear in variable_tracker?

A As a new variable row with initial undefined and updated values

B Inside person variable only

C It would not appear in variable_tracker

D As a new top-level variable

💡 Hint

Nested properties are tracked separately as seen with person.address.city in variable_tracker.

Concept Snapshot

Nested objects store objects inside other objects.
Access nested properties with dot notation: outer.inner.property.
You can read or change nested values like normal properties.
Useful for grouping related data inside one object.
Example: person.address.city accesses city inside address.

Full Transcript

We start by creating an object named person. This object has two properties: name and address. The address itself is another object with city and zip properties. When we access person.address.city, we go inside the person object to the address object, then get the city value. The code prints 'Wonderland' which is the city inside the nested address object. This shows how nested objects let us organize data inside other data. We can read or change nested properties using dot notation step by step.

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