Polymorphism through interfaces in Typescript

Introduction

Polymorphism lets us use different objects in the same way. Interfaces help us define a common shape for these objects.

When you want different objects to share the same behavior but have different details.

When you want to write code that works with many types without changing it.

When you want to make your code easier to extend with new types later.

When you want to ensure certain methods exist on different objects.

When you want to treat different objects uniformly in a collection.

Syntax

Typescript

interface InterfaceName {
  methodName(): ReturnType;
}

class ClassName implements InterfaceName {
  methodName(): ReturnType {
    // implementation
  }
}

Interfaces define what methods or properties a class must have.

Classes use implements to promise they follow the interface.

Examples

Two classes implement the same interface but have different speak sounds.

Typescript

interface Speaker {
  speak(): void;
}

class Dog implements Speaker {
  speak() {
    console.log("Woof!");
  }
}

class Cat implements Speaker {
  speak() {
    console.log("Meow!");
  }
}

Function accepts any object that implements Speaker and calls speak method.

Typescript

function makeSpeak(speaker: Speaker) {
  speaker.speak();
}

const dog = new Dog();
const cat = new Cat();

makeSpeak(dog); // Woof!
makeSpeak(cat); // Meow!

Sample Program

This program shows two classes, Car and Boat, both implementing Vehicle interface. The travel function calls move on any Vehicle.

Typescript

interface Vehicle {
  move(): void;
}

class Car implements Vehicle {
  move() {
    console.log("The car drives on the road.");
  }
}

class Boat implements Vehicle {
  move() {
    console.log("The boat sails on the water.");
  }
}

function travel(vehicle: Vehicle) {
  vehicle.move();
}

const myCar = new Car();
const myBoat = new Boat();

travel(myCar);
travel(myBoat);

Important Notes

Interfaces only describe the shape; they do not create code themselves.

Polymorphism helps keep code flexible and easy to change.

Always name interfaces clearly to show what behavior they expect.

Summary

Polymorphism lets different objects be used the same way.

Interfaces define the common methods objects must have.

Classes implement interfaces to promise they have those methods.

Practice

1. What does polymorphism through interfaces allow in TypeScript?

easy

A. Only one class can implement an interface

B. Different classes to be treated the same if they implement the same interface

C. Interfaces can hold data like variables

D. Classes cannot implement more than one interface

Solution

1. Step 1: Understand polymorphism concept

   Polymorphism means using different objects in the same way if they share a common interface.

2. Step 2: Apply to interfaces in TypeScript

   Interfaces define methods that classes promise to implement, allowing different classes to be used interchangeably.

3. Final Answer:

   Different classes to be treated the same if they implement the same interface -> Option B

4. Quick Check:

   Polymorphism = Same interface, different classes [OK]

Hint: Polymorphism means same interface, different class objects [OK]

Common Mistakes:

• Thinking interfaces hold data like variables
• Believing only one class can implement an interface
• Assuming classes can't implement multiple interfaces

2. Which of the following is the correct syntax for a TypeScript interface with a method speak() that returns a string?

easy

A. interface Animal { speak(): string; }

B. interface Animal { speak: string(); }

C. interface Animal { speak() => string; }

D. interface Animal { speak: () => string; }

Solution

1. Step 1: Review interface method syntax

   In TypeScript, methods in interfaces are declared as methodName(): returnType;.

2. Step 2: Check each option

   interface Animal { speak(): string; } correctly uses speak(): string;. interface Animal { speak: string(); } incorrectly places parentheses after the type. interface Animal { speak() => string; } uses arrow syntax wrongly. interface Animal { speak: () => string; } uses a function type, which is valid but less common for method declarations.

3. Final Answer:

   interface Animal { speak(): string; } -> Option A

4. Quick Check:

   Method syntax = methodName(): returnType [OK]

Hint: Method in interface uses colon after name, not arrow [OK]

Common Mistakes:

• Using arrow syntax inside interface method
• Putting parentheses after type instead of method name
• Confusing function type with method declaration

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

interface Shape {
  area(): number;
}

class Square implements Shape {
  constructor(private side: number) {}
  area() { return this.side * this.side; }
}

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

function printArea(shape: Shape) {
  console.log(shape.area().toFixed(2));
}

const sq = new Square(3);
const cir = new Circle(2);
printArea(sq);
printArea(cir);

medium

A. Error: area method missing

B. 6 and 12.56

C. 9.00 and 12.57

D. 9 and 12.56

Solution

1. Step 1: Calculate area of Square

   Square side is 3, so area = 3 * 3 = 9. Output with two decimals is "9.00".

2. Step 2: Calculate area of Circle

   Circle radius is 2, area = π * 2 * 2 ≈ 12.566. Rounded to two decimals is "12.57".

3. Final Answer:

   9.00 and 12.57 -> Option C

4. Quick Check:

   Square area = 9.00, Circle area ≈ 12.57 [OK]

Hint: Calculate areas, then round to 2 decimals [OK]

Common Mistakes:

• Forgetting to round with toFixed(2)
• Using wrong formula for circle area
• Confusing radius and side length

4. Identify the error in this TypeScript code using interfaces and polymorphism:

interface Vehicle {
  move(): void;
}

class Car implements Vehicle {
  move(distance: number) {
    console.log(`Car moved ${distance} meters`);
  }
}

const myCar: Vehicle = new Car();
myCar.move();

medium

A. Missing return type in move method

B. Cannot assign new Car() to Vehicle type

C. Interface Vehicle cannot have methods

D. Method move in Car has a different signature than in Vehicle interface

Solution

1. Step 1: Compare method signatures

   Interface Vehicle defines move() with no parameters. Car's move method requires a number parameter.

2. Step 2: Understand TypeScript method compatibility

   Method signatures must match exactly for interface implementation. Car's move method does not match Vehicle's move method.

3. Final Answer:

   Method move in Car has a different signature than in Vehicle interface -> Option D

4. Quick Check:

   Method signature mismatch = Error [OK]

Hint: Method signatures must match interface exactly [OK]

Common Mistakes:

• Thinking parameter difference is allowed
• Believing assignment to interface type is invalid
• Ignoring method return type requirements

5. You have an interface Logger with a method log(message: string): void. You want to create two classes: ConsoleLogger that logs to the console, and FileLogger that stores logs in an array. How can you use polymorphism to write a function writeLog(logger: Logger, msg: string) that works with both classes? Which implementation is correct?

hard

A. function writeLog(logger: Logger, msg: string) { logger.log(msg); }

B. function writeLog(logger: Logger, msg: string) { console.log(msg); }

C. function writeLog(logger: Logger, msg: string) { logger.write(msg); }

D. function writeLog(logger: Logger, msg: string) { logger.log(); }

Solution

1. Step 1: Understand polymorphism with interfaces

   The function should accept any object implementing Logger and call its log method with the message.

2. Step 2: Check each function implementation

   function writeLog(logger: Logger, msg: string) { logger.log(msg); } calls logger.log(msg) correctly. function writeLog(logger: Logger, msg: string) { console.log(msg); } ignores the passed logger and logs directly to console. function writeLog(logger: Logger, msg: string) { logger.write(msg); } calls a non-existent method write. function writeLog(logger: Logger, msg: string) { logger.log(); } calls log without arguments, which is incorrect.

3. Final Answer:

   function writeLog(logger: Logger, msg: string) { logger.log(msg); } -> Option A

4. Quick Check:

   Call interface method with correct argument [OK]

Hint: Call the interface method with the message parameter [OK]

Common Mistakes:

• Calling wrong method name
• Not passing the message argument
• Ignoring the logger parameter