Merging classes with interfaces in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of merged class and interface properties

What is the output of this TypeScript code when creating an instance of Person and accessing person.greet() and person.age?

Typescript

interface Person {
  age: number;
  greet(): string;
}

class Person {
  constructor(public age: number) {}
  greet() {
    return `Hello, I am ${this.age} years old.`;
  }
}

const person = new Person(30);
console.log(person.greet());
console.log(person.age);

A SyntaxError: Duplicate identifier 'Person'

B

"Hello, I am undefined years old."
undefined

C

"Hello, I am 30 years old."
30

D TypeError: person.greet is not a function

💡 Hint

Remember that in TypeScript, interfaces can merge with classes to add type information, but the class implementation defines the actual behavior.

The interface Person declares the shape with age and greet(). The class Person implements these members. When merged, the class provides the implementation and the interface provides the type. So, person.greet() returns the greeting string and person.age is 30.

❓ Predict Output

intermediate

Property count after merging class and interface

Given this TypeScript code, how many own properties does the employee object have after creation?

Typescript

interface Employee {
  id: number;
  department: string;
}

class Employee {
  constructor(public id: number) {}
  department = "Sales";
}

const employee = new Employee(101);
console.log(Object.keys(employee).length);

A 0

B 1

C Throws a runtime error

D 2

💡 Hint

Check which properties are assigned directly on the instance.

The class constructor assigns id as a public property, and department is initialized as a class field. Both become own properties of the employee object. So Object.keys(employee) returns ["id", "department"] which has length 2.

🔧 Debug

advanced

Why does this merged class and interface code cause an error?

This TypeScript code tries to merge a class and interface named Car. Why does it cause a compilation error?

Typescript

interface Car {
  speed: number;
  drive(): void;
}

class Car {
  speed: string;
  constructor(speed: string) {
    this.speed = speed;
  }
  drive() {
    console.log(`Driving at ${this.speed}`);
  }
}

A TypeScript error because interface and class have conflicting types for 'speed' property

B Runtime error because 'drive' method is missing

C Syntax error due to duplicate class declaration

D No error, code compiles and runs fine

💡 Hint

Check the types of the 'speed' property in both interface and class.

The interface Car declares speed as a number, but the class declares it as a string. This type conflict causes a TypeScript compilation error because merged declarations must be compatible.

📝 Syntax

advanced

Which option correctly merges a class and interface with method overloads?

You want to merge a class and interface named Calculator where the interface declares two overloads for calculate. Which option correctly compiles?

A

interface Calculator {
  calculate(x: number): number;
  calculate(x: string): string;
}
class Calculator {
  calculate(x: number): number {
    return x * 2;
  }
  calculate(x: string): string {
    return x + x;
  }
}

B

interface Calculator {
  calculate(x: number): number;
  calculate(x: string): string;
}
class Calculator {
  calculate(x: number | string): number | string {
    if (typeof x === 'number') return x * 2;
    else return x + x;
  }
}

C

interface Calculator {
  calculate(x: number): number;
  calculate(x: string): string;
}
class Calculator {
  calculate(x: any): any {
    if (typeof x === 'number') return x * 2;
    else return x + x;
  }
}

D

interface Calculator {
  calculate(x: number): number;
  calculate(x: string): string;
}
class Calculator {
  calculate(x: number | string): number {
    return x * 2;
  }
}

💡 Hint

In TypeScript, method overloads are declared in the interface, but the class must implement a single method with a compatible signature.

Option B correctly declares overloads in the interface and implements a single method in the class with a union type parameter and union return type. Option B tries to implement two methods with the same name, which is invalid. Option B lacks parameter types. Option B returns number but does not handle string input properly.

🚀 Application

expert

How many properties does this merged class/interface instance have?

Consider this TypeScript code merging a class and interface named Widget. After creating const w = new Widget();, how many own enumerable properties does w have?

Typescript

interface Widget {
  name: string;
  size: number;
  render(): void;
}

class Widget {
  name = "MyWidget";
  private size = 10;
  render() {
    console.log(`Rendering ${this.name}`);
  }
}

const w = new Widget();
console.log(Object.keys(w).length);

A 1

B 2

C 0

D 3

💡 Hint

Remember that private properties are not enumerable and do not appear in Object.keys output.

The class Widget has a public property name initialized as a class field, which is enumerable and own property. The size property is private and thus not enumerable. The method render is on the prototype, not an own property. So Object.keys(w) returns ["name"] with length 1.

Practice

1.

What happens when you declare an interface with the same name as a class in TypeScript?

For example:
class Car { drive() {} }
interface Car { color: string; }

easy

A. The interface replaces the class completely.

B. The interface merges with the class, adding the 'color' property to the class type.

C. TypeScript throws a syntax error because names conflict.

D. The class ignores the interface and remains unchanged.

Solution

1. Step 1: Understand interface and class merging

   In TypeScript, if an interface has the same name as a class, their types merge. This means the class type now includes members from both.

2. Step 2: Apply to example

   The interface adds a 'color' property to the class type, so instances of Car are expected to have 'drive()' and 'color'.

3. Final Answer:

   The interface merges with the class, adding the 'color' property to the class type. -> Option B

4. Quick Check:

   Same name interface + class = merged type [OK]

Hint: Same name means interface merges into class type [OK]

Common Mistakes:

• Thinking interface replaces class
• Expecting a syntax error
• Assuming class ignores interface

2.

Which of the following is the correct syntax to merge an interface with a class named Person in TypeScript?

easy

A. interface Person { age: number; }

B. interface Person() { age: number; }

C. interface Person[] { age: number; }

D. interface Person = { age: number; }

Solution

1. Step 1: Recall interface declaration syntax

   Interfaces are declared with the keyword 'interface' followed by the name and curly braces containing members.

2. Step 2: Check each option

   interface Person { age: number; } uses correct syntax: 'interface Person { age: number; }'. Options A, C, and D use invalid syntax like parentheses, brackets, or equals sign.

3. Final Answer:

   interface Person { age: number; } -> Option A

4. Quick Check:

   Correct interface syntax = interface Person { age: number; } [OK]

Hint: Interfaces use curly braces, no parentheses or equals [OK]

Common Mistakes:

• Using parentheses after interface name
• Using brackets instead of braces
• Assigning interface with equals sign

3.

Given the following code, what is the output?

class Animal {
  speak() { return "Roar"; }
}
interface Animal {
  age: number;
}

const a: Animal = { age: 5, speak() { return "Roar"; } };
console.log(a.age, a.speak());

medium

A. Runtime error

B. undefined Roar

C. 5 undefined

D. 5 Roar

Solution

1. Step 1: Understand merging and object literal

   The interface adds 'age' to Animal type. The object 'a' has both 'age' and 'speak' method matching Animal type.

2. Step 2: Check console.log output

   Logging 'a.age' prints 5, and 'a.speak()' returns "Roar". So output is '5 Roar'.

3. Final Answer:

   5 Roar -> Option D

4. Quick Check:

   Object has both properties, prints values [OK]

Hint: Merged type means object must have all members [OK]

Common Mistakes:

• Expecting undefined for age
• Confusing method call with property
• Thinking runtime error occurs

4.

Identify the error in this code that merges a class and interface named Book:

class Book {
  title: string;
  constructor(title: string) {
    this.title = title;
  }
}
interface Book {
  author: string;
}

const b = new Book("1984");
console.log(b.author);

medium

A. Constructor syntax is invalid.

B. Class and interface names must differ to merge.

C. Property 'author' is missing in class implementation causing runtime error.

D. No error; code runs fine.

Solution

1. Step 1: Check merging requirements

   The interface adds 'author' property to Book type, but the class does not implement it or assign it.

2. Step 2: Analyze runtime behavior

   Creating 'b' with new Book("1984") sets 'title' but 'author' is undefined. Accessing 'b.author' prints undefined, which may cause issues if expected to exist.

3. Final Answer:

   Property 'author' is missing in class implementation causing runtime error. -> Option C

4. Quick Check:

   Interface adds property, class must implement it [OK]

Hint: Merged interface members must be implemented in class [OK]

Common Mistakes:

• Thinking names must differ
• Ignoring missing property implementation
• Assuming no error occurs

5.

You want to extend a class Employee by merging an interface with the same name to add a new method getSalary(). How should you correctly implement this so that getSalary() is usable on Employee instances?

class Employee {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
}
interface Employee {
  getSalary(): number;
}

// What is the correct way to implement getSalary?

hard

A. Add getSalary() method to Employee.prototype after class declaration.

B. Add getSalary() method only in the interface; no class change needed.

C. Add getSalary() as a standalone function outside the class.

D. Add getSalary() method inside the Employee class body.

Solution

1. Step 1: Understand interface merging with class

   Merging adds type info but does not add implementation. The class must implement the method or it will cause errors.

2. Step 2: Implement method correctly

   Since the class does not have getSalary(), adding it to Employee.prototype after declaration provides the implementation and satisfies the interface.

3. Final Answer:

   Add getSalary() method to Employee.prototype after class declaration. -> Option A

4. Quick Check:

   Interface adds type; method must be implemented in class or prototype [OK]

Hint: Implement interface methods in class or prototype [OK]

Common Mistakes:

• Adding method only in interface without implementation
• Trying to add method outside prototype
• Assuming interface adds implementation automatically