Implementing interfaces in classes in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare that the class implements the interface.

Typescript

interface Animal {
  makeSound(): void;
}

class Dog implements Animal {
  makeSound() {
    console.log('Woof!');
  }
}

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

Ainherits

Bextends

Cuses

Dimplements

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'extends' instead of 'implements' for interfaces.

Forgetting to add the keyword before the interface name.

The keyword implements is used in TypeScript to declare that a class follows an interface.

2fill in blank

medium

Complete the code to add a property from the interface to the class.

Typescript

interface Vehicle {
  wheels: number;
  drive(): void;
}

class Car implements Vehicle {
  [1]: number = 4;
  drive() {
    console.log('Driving');
  }
}

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

AwheelCount

Bwheels

CnumWheels

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different property name than the interface.

Forgetting to declare the property at all.

The class must have the property named exactly as in the interface, which is wheels.

3fill in blank

hard

Fix the error by completing the method signature to match the interface.

Typescript

interface Speaker {
  speak(words: string): void;
}

class Person implements Speaker {
  speak([1]) {
    console.log(words);
  }
}

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

Awords: string

Bwords

Cmessage: number

Dtext: string

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name.

Omitting the type annotation.

Using wrong type like number instead of string.

The method parameter must match the interface exactly: name and type. Here it is words: string.

4fill in blank

hard

Fill both blanks to complete the class implementing the interface with a constructor.

Typescript

interface User {
  name: string;
  age: number;
}

class Member implements User {
  [1]: string;
  [2]: number;

  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}

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

Aname

Busername

Cage

Dyears

Attempts:

3 left

💡 Hint

Common Mistakes

Using different property names than the interface.

Forgetting to declare both properties.

The class must have properties named exactly as in the interface: name and age.

5fill in blank

hard

Fill all three blanks to implement the interface with a method and property.

Typescript

interface Logger {
  level: string;
  log(message: string): void;
}

class ConsoleLogger implements Logger {
  [1]: string = 'info';

  [2](message: string): void {
    console.log(`[$[3]] ${message}`);
  }
}

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

Alevel

Blog

Cthis.level

Dmessage

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property or method names.

Not using this to access the property inside the method.

Missing the method parameter.

The class must have the property level, the method log, and inside the method use this.level to access the property.

Practice

1. What does the implements keyword do in a TypeScript class?

easy

A. It ensures the class follows the structure defined by an interface.

B. It creates a new interface inside the class.

C. It imports an interface from another file.

D. It automatically generates methods for the class.

Solution

1. Step 1: Understand the role of interfaces

   Interfaces define a contract of properties and methods that a class must have.

2. Step 2: Role of implements keyword

   The implements keyword makes sure the class follows that contract exactly.

3. Final Answer:

   It ensures the class follows the structure defined by an interface. -> Option A

4. Quick Check:

   implements enforces interface structure [OK]

Hint: Remember: implements means 'follow the interface rules' [OK]

Common Mistakes:

• Thinking implements creates or imports interfaces
• Believing implements auto-generates code
• Confusing implements with extends

2. Which of the following is the correct syntax to make a class Car implement an interface Vehicle?

easy

A. interface Car implements Vehicle {}

B. class Car implements Vehicle {}

C. class Car extends Vehicle {}

D. class Car uses Vehicle {}

Solution

1. Step 1: Identify correct keywords for class and interface

   Classes use the keyword implements to follow interfaces.

2. Step 2: Check syntax correctness

   class Car implements Vehicle {} is the correct way to declare this.

3. Final Answer:

   class Car implements Vehicle {} -> Option B

4. Quick Check:

   Class uses implements for interfaces [OK]

Hint: Use 'implements' keyword after class name for interfaces [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Trying to implement interface in interface declaration
• Using 'uses' keyword which doesn't exist

3. What will be the output of this code?

interface Animal {
  name: string;
  speak(): string;
}

class Dog implements Animal {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  speak() {
    return `${this.name} says woof`;
  }
}

const dog = new Dog('Buddy');
console.log(dog.speak());

medium

A. undefined

B. "Dog says woof"

C. "Buddy says woof"

D. Error: Dog does not implement speak()

Solution

1. Step 1: Check class implements interface correctly

   Dog class has property name and method speak() as required.

2. Step 2: Understand method output

   speak() returns a string with the dog's name and 'says woof'.

3. Final Answer:

   "Buddy says woof" -> Option C

4. Quick Check:

   Method returns correct string with name [OK]

Hint: Look for method return values matching console.log output [OK]

Common Mistakes:

• Assuming missing method error when method exists
• Confusing property values with method output
• Ignoring string template usage

4. Identify the error in this code:

interface Shape {
  area(): number;
}

class Circle implements Shape {
  radius: number;
  constructor(radius: number) {
    this.radius = radius;
  }
  area(): string {
    return (Math.PI * this.radius * this.radius).toFixed(2);
  }
}

medium

A. Method area() returns string instead of number.

B. Missing constructor in Circle class.

C. radius property is not declared.

D. Circle should extend Shape, not implement.

Solution

1. Step 1: Check interface method signature

   Interface Shape requires area() to return a number.

2. Step 2: Compare with class method return type

   Circle's area() returns a string because toFixed() returns string.

3. Final Answer:

   Method area() returns string instead of number. -> Option A

4. Quick Check:

   Return type must match interface [OK]

Hint: Match method return types exactly with interface [OK]

Common Mistakes:

• Returning string when interface expects number
• Forgetting to declare properties
• Using extends instead of implements for interfaces

5. You have two interfaces:

interface Printer {
  print(): void;
}
interface Scanner {
  scan(): void;
}

How can you create a class AllInOneMachine that implements both interfaces correctly?

hard

A. interface AllInOneMachine implements Printer, Scanner {}

B. class AllInOneMachine implements Printer {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

C. class AllInOneMachine extends Printer, Scanner {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

D. class AllInOneMachine implements Printer, Scanner {
print() { console.log('Printing'); }
scan() { console.log('Scanning'); }
}

Solution

1. Step 1: Understand multiple interface implementation

   TypeScript allows a class to implement multiple interfaces separated by commas.

2. Step 2: Check class methods match all interfaces

   Class must have both print() and scan() methods.

3. Final Answer:

   class AllInOneMachine implements Printer, Scanner { print() { console.log('Printing'); } scan() { console.log('Scanning'); } } -> Option D

4. Quick Check:

   Multiple interfaces implemented with comma [OK]

Hint: Use comma to implement multiple interfaces in one class [OK]

Common Mistakes:

• Using extends instead of implements for interfaces
• Implementing only one interface but adding all methods
• Trying to implement interfaces in interface declaration