Polymorphism through interfaces in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is polymorphism in TypeScript?

Polymorphism means one interface can represent different types of objects. It lets you use the same method or property name for different behaviors.

beginner

How do interfaces help achieve polymorphism in TypeScript?

Interfaces define a contract with methods or properties. Different classes can implement the same interface but behave differently, enabling polymorphism.

beginner

Example: What does this interface represent?

interface Animal { speak(): void; }

It represents any animal that can speak. Different animals can implement speak() in their own way.

intermediate

What happens when you call a method on an interface type variable?

The actual method of the object assigned to that variable runs. This is polymorphism: same call, different results.

beginner

Why is polymorphism useful in programming?

It lets you write flexible and reusable code. You can work with different objects through the same interface without knowing their exact types.

What does polymorphism allow you to do with interfaces in TypeScript?

A Create only one class from an interface

B Use the same interface for different classes with different behaviors

C Prevent classes from sharing methods

D Force all classes to have the same method implementation

Polymorphism lets different classes implement the same interface but behave differently.

Which keyword is used to make a class follow an interface in TypeScript?

A inherits

B extends

C implements

D uses

The 'implements' keyword tells TypeScript that a class follows an interface.

If two classes implement the same interface, what can you do with their objects?

A Use them interchangeably where the interface type is expected

B Only use one class at a time

C Convert one class into the other automatically

D They cannot be used together

Objects of classes implementing the same interface can be used interchangeably where that interface is expected.

What will happen if a class does not implement all methods of an interface?

A TypeScript will show an error

B The class will work fine

C The interface will change automatically

D The missing methods will be ignored

TypeScript requires all interface methods to be implemented, otherwise it shows an error.

Which of these is an example of polymorphism through interfaces?

A Writing functions without parameters

B One class inherits from another without interfaces

C Using variables without types

D Different classes implement the same interface method differently

Polymorphism means different classes implement the same interface method in their own way.

Explain how polymorphism works with interfaces in TypeScript.

Think about how one interface can represent many types.

Describe a real-life example that shows polymorphism through interfaces.

Imagine animals making sounds differently but all can speak.

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