Class property declarations in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a class property declaration in TypeScript?

A class property declaration defines a variable inside a class to hold data for each object created from that class. It sets the name and type of the property.

beginner

How do you declare a public property named name of type string in a TypeScript class?

You write: <pre>class Person {
  public name: string;
}</pre> This means every Person object has a <code>name</code> property that holds a string.

intermediate

What is the difference between declaring a property with and without an access modifier in TypeScript?

If you declare a property without an access modifier, it is public by default. Using private or protected changes who can access the property.

beginner

Can you assign a default value to a class property in its declaration? Show an example.

Yes. You can assign a default value like this:<pre>class Car {
  color: string = "red";
}</pre> Every new Car object will have <code>color</code> set to "red" unless changed.

intermediate

What happens if you declare a property but do not initialize it in the class or constructor?

The property exists but is undefined until you assign a value. TypeScript may warn if you don't initialize or mark it optional.

Which keyword declares a property that can only be accessed inside the class?

A private

B public

C protected

D readonly

The private keyword restricts access to the property only within the class.

What is the default access level of a class property if no modifier is specified?

A private

B protected

C public

D readonly

Properties are public by default, meaning accessible anywhere.

How do you assign a default value to a class property in TypeScript?

A Using an equals sign in the property declaration

B Inside the constructor only

C Using a method

D You cannot assign default values

You assign default values directly in the property declaration with an equals sign.

Which of these is a valid property declaration in TypeScript?

A const age: number;

B let age: number;

C var age: number;

D age: number;

Inside a class, properties are declared without let, var, or const keywords.

What does the readonly modifier do when used with a class property?

A Makes the property private

B Prevents the property from being changed after initialization

C Allows the property to be changed anywhere

D Makes the property optional

readonly means the property can only be assigned once, usually at declaration or in the constructor.

Explain how to declare and initialize a class property with a default value in TypeScript.

Think about using an equals sign in the property line.

Describe the difference between public, private, and protected class properties in TypeScript.

Consider who can see or change the property.

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)