Method overriding with types in Typescript - Mini Project: Build & Apply

Method overriding with types

📖 Scenario: Imagine you are creating a simple system for different types of vehicles. Each vehicle can describe itself with a method. Some vehicles have extra details to share.

🎯 Goal: You will create a base class with a method, then create a subclass that overrides this method with a more specific return type.

📋 What You'll Learn

Create a base class called Vehicle with a method describe() that returns a string.

Create a subclass called Car that extends Vehicle.

Override the describe() method in Car to return a more specific type: an object with make and model properties.

Print the results of calling describe() on both Vehicle and Car instances.

💡 Why This Matters

🌍 Real World

Method overriding is common in software when different objects share behavior but need to customize details. For example, different vehicle types describe themselves differently.

💼 Career

Understanding method overriding and type safety is important for writing clear, maintainable TypeScript code in many software development jobs.

1

Create the base class Vehicle

Create a class called Vehicle with a method describe() that returns the string 'This is a vehicle.'

Typescript

# Create class Vehicle with method describe returning 'This is a vehicle.'
// Your code here

Hint

Use class Vehicle {} and inside it define describe(): string { return 'This is a vehicle.'; }

2

Create the subclass Car

Create a class called Car that extends Vehicle.

Typescript

class Vehicle {
    describe(): string {
        return 'This is a vehicle.';
    }
}

// Create class Car that extends Vehicle
// Your code here

Hint

Use class Car extends Vehicle {} to create the subclass.

3

Override describe() in Car with a specific return type

In the Car class, override the describe() method to return an object with make and model properties, both strings. For example, return { make: 'Toyota', model: 'Corolla' }.

Typescript

class Vehicle {
    describe(): string {
        return 'This is a vehicle.';
    }
}

class Car extends Vehicle {
    // Override describe() to return { make: string, model: string }
    // Your code here
}

Hint

Define describe() with return type { make: string; model: string } and return the object.

4

Print the descriptions from Vehicle and Car

Create an instance of Vehicle called vehicle and an instance of Car called car. Then print the results of vehicle.describe() and car.describe() using console.log.

Typescript

class Vehicle {
    describe(): string {
        return 'This is a vehicle.';
    }
}

class Car extends Vehicle {
    describe(): { make: string; model: string } {
        return { make: 'Toyota', model: 'Corolla' };
    }
}

// Create instances and print their describe() results
// Your code here

Hint

Use const vehicle = new Vehicle(); and const car = new Car();, then console.log(vehicle.describe()); and console.log(car.describe());.

Practice

1.

What does method overriding mean in TypeScript?

easy

A. A child class provides its own version of a method from the parent class.

B. A child class deletes a method from the parent class.

C. A parent class changes a method from the child class.

D. A method is called without any arguments.

Solution

1. Step 1: Understand method overriding

   Method overriding means a child class replaces a method defined in its parent class with its own version.

2. Step 2: Identify correct description

   A child class provides its own version of a method from the parent class. correctly describes this behavior as the child class providing its own method version.

3. Final Answer:

   A child class provides its own version of a method from the parent class. -> Option A

4. Quick Check:

   Method overriding = child method replaces parent method [OK]

Hint: Child class method replaces parent method behavior [OK]

Common Mistakes:

• Thinking parent changes child methods
• Confusing overriding with deleting methods
• Believing overriding means calling method without arguments

2.

Which of the following is the correct syntax for overriding a method greet() in a child class in TypeScript?

class Parent {
  greet(): string {
    return "Hello";
  }
}

class Child extends Parent {
  ?
}

easy

A. greet() { return 10; }

B. greet(): number { return 5; }

C. greet(name: string): string { return `Hi, ${name}`; }

D. greet(): string { return "Hi"; }

Solution

1. Step 1: Check method signature compatibility

   The child method must have the same name and compatible return type as the parent method. Parent returns string and takes no parameters.

2. Step 2: Identify correct override

   greet(): string { return "Hi"; } matches the parent's method signature exactly (no parameters, returns string). Others differ in return type or parameters.

3. Final Answer:

   greet(): string { return "Hi"; } -> Option D

4. Quick Check:

   Override method signature must match parent [OK]

Hint: Override method must match parent's name and return type [OK]

Common Mistakes:

• Changing return type to number
• Adding parameters not in parent method
• Omitting return type causing implicit any

3.

What will be the output of this TypeScript code?

class Animal {
  speak(): string {
    return "Animal sound";
  }
}

class Dog extends Animal {
  speak(): string {
    return "Bark";
  }
}

const pet: Animal = new Dog();
console.log(pet.speak());

medium

A. "Bark"

B. Error: Method not found

C. undefined

D. "Animal sound"

Solution

1. Step 1: Understand polymorphism with method overriding

   Even though the variable type is Animal, the actual object is Dog. The overridden method in Dog is called.

2. Step 2: Determine method output

   Dog's speak() returns "Bark", so console.log prints "Bark".

3. Final Answer:

   "Bark" -> Option A

4. Quick Check:

   Overridden method runs on actual object type [OK]

Hint: Actual object method runs, not variable type method [OK]

Common Mistakes:

• Thinking parent method runs due to variable type
• Expecting undefined because of type mismatch
• Assuming runtime error for overridden method

4.

Identify the error in this TypeScript code that tries to override a method:

class Vehicle {
  move(distance: number): void {
    console.log(`Moved ${distance} meters`);
  }
}

class Car extends Vehicle {
  move(distance: string): void {
    console.log(`Car moved ${distance}`);
  }
}

medium

A. Child class cannot override methods from parent class.

B. Return type void is not allowed in overriding.

C. Parameter type in child method must match parent method's parameter type.

D. Method name must be different in child class.

Solution

1. Step 1: Check method signature compatibility

   Parent method move() takes a number parameter. Child method move() takes a string parameter, which is incompatible.

2. Step 2: Identify correct error cause

   Parameter type in child method must match parent method's parameter type. correctly states this.

3. Final Answer:

   Parameter type in child method must match parent method's parameter type. -> Option C

4. Quick Check:

   Parameter types must be compatible in overriding [OK]

Hint: Parameter types must match exactly for overriding [OK]

Common Mistakes:

• Changing parameter types in child method
• Thinking return type void is invalid
• Believing method names must differ

5.

Given these classes, which override is valid and why?

class Base {
  calculate(value: number): number {
    return value * 2;
  }
}

class Derived extends Base {
  calculate(value: number): number | string {
    if (value > 10) return "Too large";
    return value * 3;
  }
}

hard

A. Valid because return type includes parent's return type (number) plus string.

B. Invalid because return type must exactly match parent's return type.

C. Invalid because method name changed.

D. Valid because child method can return any type.

Solution

1. Step 1: Check return type compatibility in overriding

   Parent method returns number only. Child method returns number or string, which is not compatible because string is not assignable to number.

2. Step 2: Understand TypeScript's strict method overriding rules

   TypeScript requires the child method's return type to be assignable to the parent's return type, not broader. Invalid because return type must exactly match parent's return type. correctly states this.

3. Final Answer:

   Invalid because return type must exactly match parent's return type. -> Option B

4. Quick Check:

   Return type must be compatible, not broader [OK]

Hint: Child return type must be assignable to parent's return type [OK]

Common Mistakes:

• Allowing broader return types in child method
• Ignoring method name consistency
• Assuming any return type is allowed in override