Method overriding with types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to override the method in the subclass.

Typescript

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

class Dog extends Animal {
  speak(): [1] {
    return "Bark";
  }
}

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

Aboolean

Bstring

Cvoid

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different return type like number or void causes a type error.

The overridden method must have the same return type 'string' as in the base class.

2fill in blank

medium

Complete the code to override the method with a parameter type.

Typescript

class Vehicle {
  move(distance: number): string {
    return `Moved ${distance} meters`;
  }
}

class Car extends Vehicle {
  move([1]): string {
    return `Car moved ${distance} meters`;
  }
}

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

Adistance: number

Bdistance: boolean

Cdistance: string

Ddistance

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the parameter type to string or boolean causes a type mismatch.

The parameter type must match the base class method's parameter type 'number'.

3fill in blank

hard

Fix the error in the method override by correcting the return type.

Typescript

class Printer {
  print(): void {
    console.log("Printing...");
  }
}

class ColorPrinter extends Printer {
  print(): [1] {
    console.log("Printing in color...");
  }
}

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

Astring

Bboolean

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the return type to string or number causes a type error.

The overridden method must have the same return type 'void' as the base class method.

4fill in blank

hard

Fill both blanks to correctly override the method with parameter and return types.

Typescript

class Calculator {
  calculate(a: number, b: number): number {
    return a + b;
  }
}

class AdvancedCalculator extends Calculator {
  calculate([1], [2]): number {
    return a * b;
  }
}

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

Aa: number

Bb: string

Cb: number

Da: string

Attempts:

3 left

💡 Hint

Common Mistakes

Using string types for parameters causes type errors.

Both parameters must have the same types as in the base class method: 'a: number' and 'b: number'.

5fill in blank

hard

Fill all three blanks to override the method with parameter types and return type.

Typescript

class Logger {
  log(message: string, level: number): [1] {
    return `${level}: ${message}`;
  }
}

class CustomLogger extends Logger {
  log([2], [3]): string {
    return `Level ${level} - ${message}`;
  }
}

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

Astring

Bmessage: string

Clevel: number

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Changing parameter types or return type causes type errors.

The return type and parameter types must match the base class method: return type 'string', parameters 'message: string' and 'level: number'.

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