Class property declarations in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a public property named name of type string.

Typescript

class Person {
  public [1]: string;
}

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

Aname

Bage

Cprivate

Dconstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Using a keyword like 'private' as the property name.

Forgetting to specify the property name.

The property name is declared as a public string property inside the class.

2fill in blank

medium

Complete the code to declare a private property named age of type number.

Typescript

class Person {
  private [1]: number;
}

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

Aname

Bage

Cpublic

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using public instead of private.

Using a wrong type like string instead of number.

The property age is declared as a private number property inside the class.

3fill in blank

hard

Fix the error in the property declaration to make isActive a boolean property.

Typescript

class User {
  isActive: [1];
}

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or number instead of boolean.

Omitting the type annotation.

The property isActive should be declared with type boolean to hold true/false values.

4fill in blank

hard

Fill both blanks to declare a readonly property id of type number.

Typescript

class Product {
  [1] [2]: number;
}

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

Areadonly

Bprivate

Cid

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using private or public instead of readonly.

Swapping the order of property name and modifier.

The property id is declared as readonly so it cannot be changed after initialization, and its type is number.

5fill in blank

hard

Fill all three blanks to declare a protected property email of type string with an initial value.

Typescript

class Account {
  protected [1]: [2] = [3];
}

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

Aemail

Bstring

C'[e-mail]'

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using number type instead of string.

Forgetting to add the initial value.

Using incorrect quotes for the string value.

The property email is declared as protected so it is accessible in subclasses, has type string, and is initialized with the value '[e-mail]'.

Practice

1. What is the purpose of declaring properties inside a TypeScript class?

easy

A. To import external modules into the class

B. To define functions that the class can use

C. To create new classes inside the current class

D. To store data related to each object created from the class

Solution

1. Step 1: Understand what class properties represent

   Class properties hold data or information specific to each object created from the class.

2. Step 2: Differentiate properties from methods and other elements

   Functions inside classes are called methods, not properties. Properties are for storing values.

3. Final Answer:

   To store data related to each object created from the class -> Option D

4. Quick Check:

   Class properties = store object data [OK]

Hint: Properties hold object data, methods hold actions [OK]

Common Mistakes:

• Confusing properties with methods
• Thinking properties create new classes
• Believing properties import modules

2. Which of the following is the correct way to declare a class property with a type and default value in TypeScript?

easy

A. class Car { speed: = 0 number; }

B. class Car { speed: number = 0; }

C. class Car { number speed = 0; }

D. class Car { speed = number 0; }

Solution

1. Step 1: Recall TypeScript property syntax

   Properties are declared as name: type = defaultValue;.

2. Step 2: Check each option for correct syntax

   class Car { speed: number = 0; } matches the correct syntax: speed: number = 0;. Others have incorrect order or missing punctuation.

3. Final Answer:

   class Car { speed: number = 0; } -> Option B

4. Quick Check:

   Property syntax = name: type = value [OK]

Hint: Use colon for type, equals for default value [OK]

Common Mistakes:

• Swapping type and value order
• Missing colon between name and type
• Using wrong punctuation

3. What will be the output of this TypeScript code?

class Person {
  name: string = "Alice";
  age: number = 30;
}
const p = new Person();
console.log(p.name + ' is ' + p.age + ' years old');

medium

A. "Alice is 30 years old"

B. "undefined is undefined years old"

C. Error: Property 'name' does not exist

D. "Alice is undefined years old"

Solution

1. Step 1: Understand property initialization

   The class Person has properties name and age initialized with "Alice" and 30 respectively.

2. Step 2: Check the output of console.log

   Creating an object p sets these properties. The log concatenates them into the string "Alice is 30 years old".

3. Final Answer:

   "Alice is 30 years old" -> Option A

4. Quick Check:

   Initialized properties print correct values [OK]

Hint: Default values print as expected when object created [OK]

Common Mistakes:

• Assuming properties are undefined without constructor
• Confusing property names or types
• Expecting runtime errors without syntax mistakes

4. Identify the error in this TypeScript class property declaration:

class Book {
  title string = "Unknown";
  pages: number = 100;
}

medium

A. Class name should be lowercase

B. Missing default value for 'pages' property

C. Missing colon ':' between property name and type in 'title string = "Unknown";'

D. Properties cannot have default values

Solution

1. Step 1: Check syntax of property declarations

   Property declarations require a colon ':' between the name and type, like title: string.

2. Step 2: Identify the incorrect line

   The line title string = "Unknown"; misses the colon, causing a syntax error.

3. Final Answer:

   Missing colon ':' between property name and type in 'title string = "Unknown";' -> Option C

4. Quick Check:

   Property syntax requires colon ':' [OK]

Hint: Always put ':' between property name and type [OK]

Common Mistakes:

• Omitting colon between name and type
• Thinking default values are not allowed
• Ignoring case sensitivity of class names (not an error)

5. You want to create a TypeScript class Student with a property grades that stores an array of numbers and defaults to an empty array. Which is the correct declaration to ensure each object has its own separate array?

hard

A. class Student { grades: number[] = []; }

B. class Student { static grades: number[] = []; }

C. class Student { grades: number[]; } // no default value

D. class Student { grades = []; }

Solution

1. Step 1: Understand instance vs static properties

   Instance properties belong to each object separately; static properties are shared by all objects.

2. Step 2: Check which option creates separate arrays per object

   class Student { grades: number[] = []; } declares grades as an instance property with default empty array, so each object gets its own array.

3. Step 3: Analyze other options

   class Student { static grades: number[] = []; } uses static, so all objects share one array (not desired). class Student { grades: number[]; } // no default value has no default, so array is undefined unless set later. class Student { grades = []; } misses type annotation explicitly, inferring any[] instead of number[].

4. Final Answer:

   class Student { grades: number[] = []; } -> Option A

5. Quick Check:

   Instance property with default array = separate arrays [OK]

Hint: Use instance property with default [] for separate arrays [OK]

Common Mistakes:

• Using static for properties needing separate values
• Not initializing arrays causing undefined errors
• Omitting type annotation (less clear)