Extending classes with types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to extend a class in TypeScript?

Extending a class means creating a new class that inherits properties and methods from an existing class. This helps reuse code and add new features.

beginner

How do you specify the type of a class property when extending a class?

You declare the property with its type in the class definition. When extending, you can keep the same type or specify a more specific type if needed.

beginner

What is the syntax to extend a class named <code>Animal</code> in TypeScript?

Use <code>class Dog extends Animal { }</code> to create a new class <code>Dog</code> that inherits from <code>Animal</code>.

intermediate

Can a subclass override a method from its superclass in TypeScript? How?

Yes, a subclass can provide its own version of a method by defining a method with the same name. This replaces the superclass method when called on the subclass instance.

intermediate

How do you call the constructor of a superclass inside a subclass in TypeScript?

Use the <code>super()</code> function inside the subclass constructor to call the superclass constructor and pass any required arguments.

What keyword is used to create a class that inherits from another class in TypeScript?

A inherits

B implements

C super

D extends

The keyword extends is used to create a subclass that inherits from a superclass.

How do you specify the type of a property in a TypeScript class?

A propertyName: type;

B propertyName = type;

C type propertyName;

D propertyName -> type;

In TypeScript, you specify a property type using propertyName: type; syntax.

Which method allows a subclass to use the constructor of its superclass?

A this()

B super()

C parent()

D base()

The super() function calls the superclass constructor inside the subclass constructor.

If a subclass defines a method with the same name as its superclass, what happens?

A The subclass method overrides the superclass method.

B Both methods run automatically.

C The superclass method is called instead.

D An error occurs.

The subclass method replaces the superclass method when called on subclass instances.

Can a subclass add new properties not present in the superclass?

A No, subclasses must have the same properties.

B Only if the superclass allows it.

C Yes, subclasses can add new properties.

D Only if the subclass is abstract.

Subclasses can add new properties and methods beyond what the superclass has.

Explain how extending classes works in TypeScript and why it is useful.

Think about how a new class can get features from an existing class.

Describe how to override a method in a subclass and how to call the superclass constructor.

Focus on how subclasses change behavior and initialize.

Practice

1. What does the extends keyword do in TypeScript class definitions?

easy

A. It imports a module into the class.

B. It defines a new interface for a class.

C. It creates a new class that inherits properties and methods from an existing class.

D. It declares a variable inside a class.

Solution

1. Step 1: Understand the role of extends

   The extends keyword is used to create a new class that inherits from an existing class, gaining its properties and methods.

2. Step 2: Differentiate from other keywords

   Other options describe unrelated concepts: interfaces, variables, or modules, which are not what extends does.

3. Final Answer:

   It creates a new class that inherits properties and methods from an existing class. -> Option C

4. Quick Check:

   extends means inheritance [OK]

Hint: Remember: extends means 'is a' relationship in classes [OK]

Common Mistakes:

• Confusing extends with interface implementation
• Thinking extends imports modules
• Using extends to declare variables

2. Which of the following is the correct syntax to extend a class named Animal in TypeScript?

easy

A. class Dog extends Animal {}

B. class Dog inherits Animal {}

C. class Dog implements Animal {}

D. class Dog import Animal {}

Solution

1. Step 1: Recall correct syntax for class extension

   In TypeScript, the keyword to extend a class is extends, so the syntax is class Dog extends Animal {}.

2. Step 2: Identify incorrect options

   inherits is not a TypeScript keyword, implements is for interfaces, and import is for modules.

3. Final Answer:

   class Dog extends Animal {} -> Option A

4. Quick Check:

   Use extends to inherit classes [OK]

Hint: Use 'extends' keyword to inherit classes, not 'inherits' or 'implements' [OK]

Common Mistakes:

• Using 'inherits' instead of 'extends'
• Confusing 'implements' with 'extends'
• Trying to import classes inside class declaration

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

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet() {
    return `Hello, ${this.name}`;
  }
}

class Student extends Person {
  constructor(name: string) {
    super(name);
  }
}

const s = new Student('Anna');
console.log(s.greet());

medium

A. Hello, undefined

B. Hello, Anna

C. Error: super() not called

D. Hello, Student

Solution

1. Step 1: Understand class inheritance and constructor call

   The Student class extends Person and calls super(name) in its constructor, so the name property is set correctly.

2. Step 2: Check method call and output

   Calling s.greet() uses the inherited method, which returns Hello, Anna.

3. Final Answer:

   Hello, Anna -> Option B

4. Quick Check:

   super() sets parent properties correctly [OK]

Hint: Always call super() in child constructor to set parent properties [OK]

Common Mistakes:

• Forgetting to call super() causes errors
• Expecting child's own greet method without override
• Assuming name is undefined without super()

4. Identify the error in this TypeScript code:

class Vehicle {
  wheels: number;
  constructor(wheels: number) {
    this.wheels = wheels;
  }
}

class Car extends Vehicle {
  constructor() {
    this.wheels = 4;
  }
}

medium

A. Missing call to super() in Car constructor

B. Cannot assign number to wheels property

C. Class Car should not extend Vehicle

D. Constructor syntax is incorrect

Solution

1. Step 1: Check constructor in subclass

   The Car class constructor assigns this.wheels = 4 but does not call super(), which is required before using this.

2. Step 2: Understand TypeScript rules for subclass constructors

   In TypeScript, a subclass constructor must call super() before accessing this. Missing this causes a compile error.

3. Final Answer:

   Missing call to super() in Car constructor -> Option A

4. Quick Check:

   Always call super() before this in subclass constructor [OK]

Hint: Call super() first in subclass constructor before using this [OK]

Common Mistakes:

• Assigning to this before super() call
• Forgetting super() call causes runtime error
• Thinking super() is optional in subclass constructor

5. Given these classes, how can you extend Employee to add a department property and keep the name property from Person?

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
}

class Employee extends Person {
  // Add department property here
}

hard

A. class Employee extends Person {
department: string;
constructor(name: string, department: string) {
this.department = department;
}
}

B. class Employee extends Person {
department: string;
constructor(department: string) {
this.department = department;
}
}

C. class Employee extends Person {
department: string;
constructor(name: string) {
super(name);
}
}

D. class Employee extends Person {
department: string;
constructor(name: string, department: string) {
super(name);
this.department = department;
}
}

Solution

1. Step 1: Add department property and constructor parameters

   To add department, declare it and add it as a parameter in the constructor along with name.

2. Step 2: Call super(name) and assign department

   Call super(name) to set the inherited name property, then assign this.department = department.

3. Final Answer:

   class Employee extends Person { department: string; constructor(name: string, department: string) { super(name); this.department = department; } } -> Option D

4. Quick Check:

   Use super() for parent props and assign new props in constructor [OK]

Hint: Call super(name) then assign new properties in constructor [OK]

Common Mistakes:

• Not calling super() in constructor
• Missing department parameter in constructor
• Assigning this.department before super() call