Class implementing multiple interfaces in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class implementing two interfaces

What will be the output of the following TypeScript code?

Typescript

interface A {
  greet(): string;
}

interface B {
  farewell(): string;
}

class C implements A, B {
  greet() {
    return "Hello";
  }
  farewell() {
    return "Goodbye";
  }
}

const obj = new C();
console.log(obj.greet() + ", " + obj.farewell());

A "Goodbye, Hello"

B "Hello, Goodbye"

C TypeError at runtime

D SyntaxError at compile time

💡 Hint

Check the methods implemented by class C and their return values.

Class C implements both interfaces A and B by providing greet() and farewell() methods. Calling greet() returns "Hello" and farewell() returns "Goodbye", so the combined output is "Hello, Goodbye".

❓ Predict Output

intermediate

Value of property after implementing multiple interfaces

Given the following code, what is the value of obj.status after execution?

Typescript

interface X {
  status: string;
}

interface Y {
  status: string;
}

class Z implements X, Y {
  status = "active";
}

const obj = new Z();

A "active"

B "inactive"

C undefined

D Compilation error due to duplicate property

💡 Hint

Consider how TypeScript handles properties with the same name in multiple interfaces.

Both interfaces declare a property named status of type string. The class Z implements both and defines status as "active". This is valid and obj.status is "active".

🔧 Debug

advanced

Identify the error in class implementing multiple interfaces

What error will this TypeScript code produce?

Typescript

interface First {
  doSomething(): void;
}

interface Second {
  doSomething(): number;
}

class Test implements First, Second {
  doSomething() {
    return 42;
  }
}

A No error, code compiles and runs fine.

B Syntax error: Missing return type in method.

C Runtime error: doSomething returns wrong type.

D TypeScript error: Class incorrectly implements interface 'First'. Type 'number' is not assignable to type 'void'.

💡 Hint

Check the return types of the method in both interfaces and the class.

The method doSomething() in interface First returns void, but in Second it returns number. The class method returns number, which conflicts with First's void return type, causing a TypeScript error.

📝 Syntax

advanced

Correct syntax for implementing multiple interfaces

Which of the following is the correct syntax for a class implementing interfaces Alpha and Beta in TypeScript?

A class MyClass implements Alpha, Beta {}

B class MyClass implements (Alpha & Beta) {}

C class MyClass implements [Alpha, Beta] {}

D class MyClass extends Alpha, Beta {}

💡 Hint

Remember how TypeScript lists multiple interfaces in the implements clause.

The correct syntax uses a comma-separated list of interfaces after implements. Using & or arrays or extends with multiple interfaces is invalid syntax.

🚀 Application

expert

Determine the number of methods in a class implementing multiple interfaces

Given these interfaces and class, how many methods does the class FinalClass have?

Typescript

interface One {
  methodA(): void;
  methodB(): void;
}

interface Two {
  methodB(): void;
  methodC(): void;
}

interface Three {
  methodD(): void;
}

class FinalClass implements One, Two, Three {
  methodA() {}
  methodB() {}
  methodC() {}
  methodD() {}
}

A 5

B 3

C 4

D 2

💡 Hint

Count unique method names implemented in the class.

The class implements all methods from the three interfaces. methodB appears in two interfaces but is implemented once. Total unique methods are methodA, methodB, methodC, methodD = 4.