Why objects are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an object representing a person with a name.

Javascript

const person = { name: [1] }; console.log(person.name);

Drag options to blanks, or click blank then click option'

AAlice

B'Alice'

Cname

Dperson

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the string value.

Using a variable name instead of a string.

The name value must be a string, so it needs quotes around it.

2fill in blank

medium

Complete the code to access the age property of the object.

Javascript

const user = { age: 30 }; console.log(user.[1]);

Drag options to blanks, or click blank then click option'

Aname

Buser

C30

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using the value instead of the property name.

Using a wrong property name.

To get the age, use the property name 'age' after the dot.

3fill in blank

hard

Fix the error in the code to correctly add a new property to the object.

Javascript

const car = {}; car[1]color = 'red'; console.log(car.color);

Drag options to blanks, or click blank then click option'

A.

B[

C=

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of '.' before the property name.

Using brackets without quotes.

Use dot notation to add a new property to an object.

4fill in blank

hard

Fill both blanks to create an object with a method that returns a greeting.

Javascript

const greeter = { greet() { return [1] + this.[2]; } , name: 'Bob' }; console.log(greeter.greet());

Drag options to blanks, or click blank then click option'

A'Hello, '

Bname

Cgreet

D'Hi, '

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the greeting string.

Using the wrong property name after 'this'.

The method returns a greeting string plus the object's name property.

5fill in blank

hard

Fill all three blanks to create an object with properties and a method that uses them.

Javascript

const book = { title: [1], author: [2], getSummary() { return `${this.[3] by ${this.author}`; } }; console.log(book.getSummary());

Drag options to blanks, or click blank then click option'

A'1984'

B'George Orwell'

Ctitle

Dauthor

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up property names inside the method.

Not using 'this' to access object properties.

The method uses the title and author properties to create a summary string.

Practice

1. Why do we use objects in JavaScript?
const person = { name: 'Alice', age: 25 };

easy

A. To group related data under one name

B. To perform mathematical calculations

C. To create loops

D. To write comments

Solution

1. Step 1: Understand the purpose of objects

   Objects group related data, like name and age, under one variable.

2. Step 2: Compare with other options

   Options B, C, and D describe other programming tasks, not grouping data.

3. Final Answer:

   To group related data under one name -> Option A

4. Quick Check:

   Objects group data = A [OK]

Hint: Objects hold related info together [OK]

Common Mistakes:

• Thinking objects are for calculations
• Confusing objects with loops
• Believing objects are for comments

2. Which of the following is the correct way to create an object in JavaScript?

easy

A. const obj = [name: 'John', age: 30];

B. const obj = (name: 'John', age: 30);

C. const obj = { name: 'John', age: 30 };

D. const obj = ;

Solution

1. Step 1: Recall object syntax in JavaScript

   Objects use curly braces {} with key: value pairs inside.

2. Step 2: Check each option

   const obj = { name: 'John', age: 30 }; uses correct curly braces and key-value pairs syntax.

3. Final Answer:

   const obj = { name: 'John', age: 30 }; -> Option C

4. Quick Check:

   Curly braces with key:value = A [OK]

Hint: Objects use curly braces with key:value pairs [OK]

Common Mistakes:

• Using parentheses instead of braces
• Using square brackets for objects
• Using angle brackets incorrectly

3. What will be the output of this code?

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

medium

A. car.brand

B. 2020

C. undefined

D. 'Toyota'

Solution

1. Step 1: Access the object property

   car.brand accesses the value of the brand key in the car object.

2. Step 2: Identify the value stored

   The brand key has the value 'Toyota', so console.log outputs 'Toyota'.

3. Final Answer:

   'Toyota' -> Option D

4. Quick Check:

   car.brand = 'Toyota' [OK]

Hint: Access object properties with dot notation [OK]

Common Mistakes:

• Printing the key name instead of value
• Confusing property access syntax
• Expecting the year instead of brand

4. Find the error in this object creation:

const book = { title: 'JS Guide', pages = 300 };

medium

A. Using '=' instead of ':' for key-value

B. Object keys must be strings

C. Missing comma between properties

D. Missing semicolon after object

Solution

1. Step 1: Check key-value syntax in object

   Object properties use colon ':' to separate key and value, not '='.

2. Step 2: Identify the error in code

   The code uses pages = 300, which is incorrect syntax for objects.

3. Final Answer:

   Using '=' instead of ':' for key-value -> Option A

4. Quick Check:

   Key:value uses ':' not '=' [OK]

Hint: Use colon ':' between key and value in objects [OK]

Common Mistakes:

• Using '=' instead of ':'
• Forgetting commas between properties
• Thinking keys must be quoted always

5. You want to store information about a student including name, age, and grades for math and science. Which is the best way to organize this data using objects?

hard

A. const student = ['John', 20, 90, 85];

B. const student = { name: 'John', age: 20, grades: { math: 90, science: 85 } };

C. const student = { 'John', 20, 90, 85 };

D. const student = { name = 'John', age = 20, math = 90, science = 85 };

Solution

1. Step 1: Identify how to group related data

   Use an object with keys for name, age, and a nested object for grades.

2. Step 2: Check each option's structure

   const student = { name: 'John', age: 20, grades: { math: 90, science: 85 } }; correctly nests grades inside the student object with proper syntax.

3. Final Answer:

   const student = { name: 'John', age: 20, grades: { math: 90, science: 85 } }; -> Option B

4. Quick Check:

   Nested objects organize related info best [OK]

Hint: Use nested objects for grouped related data [OK]

Common Mistakes:

• Using arrays without keys for mixed data
• Incorrect object syntax with '='
• Not grouping grades inside an object