Accessing object properties 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 user = { name: 'Alice', details: { age: 30, city: 'Paris' } };
console.log(user.details.city);

A "Paris"

B "city"

C undefined

D TypeError

💡 Hint

Think about how to access properties inside nested objects using dot notation.

The property 'city' is inside the 'details' object. Accessing user.details.city returns the string 'Paris'.

❓ Predict Output

intermediate

Accessing object properties with bracket notation

What will this code output?

Javascript

const obj = { 'first-name': 'John', age: 25 };
console.log(obj['first-name']);

A "first-name"

B undefined

C "John"

D SyntaxError

💡 Hint

Use bracket notation to access properties with special characters or hyphens.

Since 'first-name' contains a hyphen, it must be accessed with bracket notation. obj['first-name'] returns 'John'.

❓ Predict Output

advanced

Accessing properties with variables

What is the output of this code?

Javascript

const key = 'score';
const player = { name: 'Bob', score: 42 };
console.log(player[key]);

A "score"

B 42

C undefined

D ReferenceError

💡 Hint

When using a variable to access a property, use bracket notation.

player[key] accesses the property named 'score' because key holds the string 'score'. The value is 42.

❓ Predict Output

advanced

Optional chaining with object properties

What will this code output?

Javascript

const data = { user: { profile: { name: 'Eve' } } };
console.log(data.user?.profile?.age);

A "age"

B null

C TypeError

D undefined

💡 Hint

Optional chaining returns undefined if a property does not exist instead of throwing an error.

The property 'age' does not exist inside profile, so data.user?.profile?.age returns undefined safely.

❓ Predict Output

expert

Dynamic property access and computed property names

What is the output of this code?

Javascript

const prefix = 'user';
const id = 7;
const obj = { [`${prefix}${id}`]: 'active' };
console.log(obj.user7);

A "active"

B SyntaxError

C undefined

D TypeError

💡 Hint

Computed property names allow dynamic keys in object literals.

The key is computed as 'user7', so obj.user7 returns 'active'.

Practice

1. Which notation is used to access a property named age in a JavaScript object person when the property name is known and simple?

easy

A. person->age

B. person.age

C. person['age']

D. person(age)

Solution

1. Step 1: Understand dot notation usage

   Dot notation is used when the property name is a simple, valid identifier known in advance.

2. Step 2: Apply dot notation to access property

   Using person.age accesses the age property directly.

3. Final Answer:

   person.age -> Option B

4. Quick Check:

   Dot notation = person.age [OK]

Hint: Use dot notation for simple known property names [OK]

Common Mistakes:

• Using arrow (->) instead of dot
• Using parentheses like a function call
• Using bracket notation unnecessarily

2. Which of the following is the correct syntax to access the property "first-name" of an object user?

easy

A. user['first-name']

B. user.first-name

C. user.first_name

D. user[first-name]

Solution

1. Step 1: Identify property name with special characters

   The property name "first-name" contains a hyphen, which is not valid for dot notation.

2. Step 2: Use bracket notation with quotes

   Bracket notation with quotes, user['first-name'], correctly accesses such properties.

3. Final Answer:

   user['first-name'] -> Option A

4. Quick Check:

   Bracket notation for special chars = user['first-name'] [OK]

Hint: Use bracket notation for property names with special characters [OK]

Common Mistakes:

• Using dot notation with hyphenated names
• Omitting quotes inside brackets
• Using undefined variable inside brackets

3. What will be the output of the following code?

const car = { brand: 'Toyota', year: 2020 };
console.log(car['brand']);

medium

A. Error

B. brand

C. undefined

D. Toyota

Solution

1. Step 1: Understand bracket notation access

   The code uses bracket notation with the string 'brand' to access the property value.

2. Step 2: Retrieve the value of 'brand'

   The value of car['brand'] is 'Toyota', so it prints 'Toyota'.

3. Final Answer:

   Toyota -> Option D

4. Quick Check:

   car['brand'] = 'Toyota' [OK]

Hint: Bracket notation with string key returns property value [OK]

Common Mistakes:

• Confusing property name with value
• Expecting property name to print
• Using wrong quotes causing error

4. Identify the error in the following code snippet:

const book = { title: '1984', author: 'Orwell' };
console.log(book[title]);

medium

A. Syntax error due to missing semicolon

B. Dot notation should be used instead

C. Missing quotes around 'title' in bracket notation

D. Property 'title' does not exist

Solution

1. Step 1: Check bracket notation usage

   Inside brackets, property names must be strings or variables. Here, title is treated as a variable, not a string.

2. Step 2: Fix by adding quotes

   Adding quotes: book['title'] correctly accesses the property.

3. Final Answer:

   Missing quotes around 'title' in bracket notation -> Option C

4. Quick Check:

   Bracket keys need quotes if not variables [OK]

Hint: Put quotes around property names in brackets unless variable [OK]

Common Mistakes:

• Forgetting quotes around string keys
• Assuming dot notation always required
• Confusing variable and string keys

5. Given the object settings = { 'theme-color': 'dark', fontSize: 14 }, which code correctly changes the theme-color to 'light' and increases fontSize by 2?

hard

A. settings['theme-color'] = 'light'; settings.fontSize += 2;

B. settings.theme-color = 'light'; settings['fontSize'] += 2;

C. settings.theme-color = 'light'; settings.fontSize = settings.fontSize + 2;

D. settings['theme-color'] = 'light'; settings['fontSize'] =+ 2;

Solution

1. Step 1: Update 'theme-color' using bracket notation

   Since 'theme-color' has a hyphen, use bracket notation: settings['theme-color'] = 'light';

2. Step 2: Increase fontSize using dot notation

   fontSize is a simple property, so settings.fontSize += 2; correctly adds 2.

3. Final Answer:

   settings['theme-color'] = 'light'; settings.fontSize += 2; -> Option A

4. Quick Check:

   Bracket for special keys + dot for simple keys [OK]

Hint: Use bracket for special keys, dot for simple keys [OK]

Common Mistakes:

• Using dot notation for hyphenated keys
• Mistyping += as =+
• Mixing up property names and strings