Generic factory pattern in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this generic factory function?

Consider the following TypeScript code using a generic factory pattern. What will be printed to the console?

Typescript

function createInstance<T>(ctor: new () => T): T {
  return new ctor();
}

class Dog {
  speak() { return "Woof!"; }
}

const dog = createInstance(Dog);
console.log(dog.speak());

A "Woof!"

B undefined

C TypeError at runtime

D Compilation error

💡 Hint

The factory function creates an instance using the constructor passed in. What does dog.speak() return?

The createInstance function takes a class constructor and returns a new instance. Since Dog has a speak method returning "Woof!", the console prints "Woof!".

🧠 Conceptual

intermediate

Which statement about generic factory pattern is true?

Choose the correct statement about the generic factory pattern in TypeScript.

A It allows creating instances of any class type passed as a constructor parameter.

B It requires the class to implement a specific interface with a factory method.

C It can only create instances of classes without constructor parameters.

D It cannot be used with classes that have private constructors.

💡 Hint

Think about how generics and constructors work in TypeScript.

The generic factory pattern uses a constructor signature new () => T to create instances of any class type passed in. It does not require interfaces or factory methods, but it does require a constructor without parameters.

🔧 Debug

advanced

Why does this generic factory code cause a compilation error?

Examine the following TypeScript code. Why does it cause a compilation error?

Typescript

function createInstance<T>(ctor: new (name: string) => T, name: string): T {
  return new ctor();
}

class Cat {
  constructor(public name: string) {}
}

const cat = createInstance(Cat, "Whiskers");

A The class Cat does not have a default constructor.

B The generic type T is not specified explicitly.

C The constructor is called without the required 'name' argument.

D The function createInstance cannot accept parameters.

💡 Hint

Look at how the constructor is called inside the function.

The constructor signature requires a string argument, but new ctor() is called without arguments, causing a compilation error.

📝 Syntax

advanced

Which option correctly types a generic factory function for classes with parameters?

Choose the correct TypeScript signature for a generic factory function that can create instances of classes with any constructor parameters.

A function createInstance<T>(ctor: new (...args: T[]) => T): T

B function createInstance<T>(ctor: new () => T, args: any[]): T

C function createInstance<T>(ctor: T, ...args: any[]): T

D function createInstance<T>(ctor: new (...args: any[]) => T, ...args: any[]): T

💡 Hint

Consider how to type a constructor with any number of parameters.

Option D correctly uses a constructor signature with rest parameters new (...args: any[]) => T and passes the arguments to create instances with any constructor parameters.

🚀 Application

expert

How many items are in the resulting map after using this generic factory?

Given the following TypeScript code using a generic factory to create multiple instances, how many entries does the instances map contain?

Typescript

class User {
  constructor(public id: number) {}
}

class Product {
  constructor(public id: number) {}
}

function createInstance<T>(ctor: new (id: number) => T, id: number): T {
  return new ctor(id);
}

const instances = new Map<string, unknown>();
instances.set("user1", createInstance(User, 1));
instances.set("user2", createInstance(User, 2));
instances.set("product1", createInstance(Product, 1));
instances.set("user1", createInstance(User, 3));

A 1

B 3

C 2

D 4

💡 Hint

Remember how Map keys work when adding entries with the same key.

The key "user1" is used twice. The second set call overwrites the first. So the map has keys: "user1", "user2", "product1" → 3 entries.