Generic factory pattern in Typescript

Introduction

The generic factory pattern helps create objects without specifying exact classes. It makes code flexible and reusable.

When you want to create different types of objects using the same code.

When you need to add new object types without changing existing code.

When you want to separate object creation from usage.

When you want to manage object creation logic in one place.

When you want to use type safety with different object types.

Syntax

Typescript

interface Factory<T> {
  create(): T;
}

class ConcreteFactory<T> implements Factory<T> {
  constructor(private creator: () => T) {}
  create(): T {
    return this.creator();
  }
}

The Factory interface defines a method to create objects of type T.

The ConcreteFactory class takes a function that creates objects and uses it to produce new instances.

Examples

This interface says any factory must have a create method that returns an object of type T.

Typescript

interface Factory<T> {
  create(): T;
}

This class uses a function passed in the constructor to create objects of type T.

Typescript

class ConcreteFactory<T> implements Factory<T> {
  constructor(private creator: () => T) {}
  create(): T {
    return this.creator();
  }
}

Here, we create a factory for Dog objects and use it to make a dog that can speak.

Typescript

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

const dogFactory = new ConcreteFactory(() => new Dog());
const dog = dogFactory.create();
console.log(dog.speak());

Sample Program

This program defines a generic factory pattern to create Cat and Dog objects. Each factory uses a function to create the right animal. Then it calls speak() on each animal.

Typescript

interface Factory<T> {
  create(): T;
}

class ConcreteFactory<T> implements Factory<T> {
  constructor(private creator: () => T) {}
  create(): T {
    return this.creator();
  }
}

class Cat {
  speak() { return 'Meow!'; }
}

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

const catFactory = new ConcreteFactory(() => new Cat());
const dogFactory = new ConcreteFactory(() => new Dog());

const cat = catFactory.create();
const dog = dogFactory.create();

console.log(cat.speak());
console.log(dog.speak());

Important Notes

You can use arrow functions or normal functions as creators.

This pattern helps keep your code clean and easy to extend.

Using generics ensures type safety when creating objects.

Summary

The generic factory pattern creates objects without knowing their exact class.

It uses a generic interface and a creator function to make objects.

This pattern makes your code flexible and easy to add new types.

Practice

1.

What is the main purpose of the Generic factory pattern in TypeScript?

easy

A. To enforce strict typing on all variables.

B. To create objects without specifying their exact class.

C. To handle asynchronous operations automatically.

D. To optimize code for faster execution.

Solution

1. Step 1: Understand the pattern's goal

   The generic factory pattern is designed to create objects without needing to know their exact class ahead of time.

2. Step 2: Compare options with the pattern's purpose

   To create objects without specifying their exact class. correctly states this purpose. Other options describe unrelated features.

3. Final Answer:

   To create objects without specifying their exact class. -> Option B

4. Quick Check:

   Generic factory pattern = create objects flexibly [OK]

Hint: Focus on object creation without class details [OK]

Common Mistakes:

• Confusing with async handling
• Thinking it enforces variable typing
• Assuming it optimizes runtime speed

2.

Which of the following is the correct syntax to declare a generic factory function in TypeScript?

function create<T>(): T { /* ... */ }

easy

A. function create<T>(): T { return new T(); }

B. function create<T>(): T { return T(); }

C. function create<T>(): T { return new Object(); }

D. function create<T>(): T { return {} as T; }

Solution

1. Step 1: Analyze generic function syntax

   The function declares a generic type T and returns an object of type T.

2. Step 2: Check return statements for correctness

   function create<T>(): T { return {} as T; } uses a type assertion {} as T which is valid to create a generic placeholder object. function create<T>(): T { return new T(); } tries to use new T() which is invalid because T is a type parameter, not a constructor. function create<T>(): T { return T(); } calls T() like a function which is invalid. function create<T>(): T { return new Object(); } returns a generic object but not typed as T.

3. Final Answer:

   function create<T>(): T { return {} as T; } -> Option D

4. Quick Check:

   Generic factory syntax = use type assertion [OK]

Hint: Use type assertion to return generic type instance [OK]

Common Mistakes:

• Trying to instantiate generic type with new
• Calling generic type as a function
• Returning untyped objects without assertion

3.

What will be the output of this TypeScript code using a generic factory pattern?

interface Animal { speak(): string; }
class Dog implements Animal { speak() { return "Woof"; } }
function createAnimal<T extends Animal>(ctor: new () => T): T {
  return new ctor();
}
const dog = createAnimal(Dog);
console.log(dog.speak());

medium

A. "Woof"

B. undefined

C. TypeError at runtime

D. Compilation error

Solution

1. Step 1: Understand the factory function behavior

   The function createAnimal takes a constructor ctor and returns a new instance of type T which extends Animal.

2. Step 2: Trace the code execution

   We pass Dog class to createAnimal, so it returns a new Dog instance. Calling dog.speak() returns "Woof".

3. Final Answer:

   "Woof" -> Option A

4. Quick Check:

   Generic factory creates Dog, speak() = "Woof" [OK]

Hint: Trace constructor call and method output [OK]

Common Mistakes:

• Assuming generic type can't be instantiated
• Confusing type constraints with runtime errors
• Expecting undefined instead of method return

4.

Identify the error in this generic factory function and select the fix.

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

medium

A. Add a constructor parameter: ctor: new () => T and return new ctor().

B. Remove new keyword and call T() directly.

C. Change return type to any to avoid errors.

D. Use Object.create(T) instead of new T().

Solution

1. Step 1: Identify why new T() causes error

   TypeScript does not allow new T() because T is a type parameter, not a constructor function.

2. Step 2: Apply correct fix using constructor parameter

   Passing a constructor function ctor: new () => T allows calling new ctor() to create an instance of T.

3. Final Answer:

   Add a constructor parameter: ctor: new () => T and return new ctor(). -> Option A

4. Quick Check:

   Generic factory needs constructor parameter [OK]

Hint: Pass constructor as parameter to instantiate generic type [OK]

Common Mistakes:

• Trying to instantiate generic type directly
• Removing new keyword incorrectly
• Changing return type to any loses type safety

5.

You want to create a generic factory that can produce different shapes with a draw() method. Which approach correctly uses the generic factory pattern to support adding new shapes easily?

interface Shape { draw(): void; }
class Circle implements Shape { draw() { console.log("Circle"); } }
class Square implements Shape { draw() { console.log("Square"); } }
// What is the best factory implementation?

hard

A. function createShape<T>(): T { return {} as T; }

B. function createShape(): Shape { return new Circle(); }

C. function createShape(ctor: new () => T): T { return new ctor(); }

D. function createShape(type: string): Shape { if(type === 'circle') return new Circle(); else return new Square(); }

Solution

1. Step 1: Understand the requirement for flexibility

   The factory should create any shape implementing Shape without hardcoding types.

2. Step 2: Evaluate each option

   function createShape(ctor: new () => T): T { return new ctor(); } uses a generic constructor parameter allowing easy addition of new shapes. function createShape(): Shape { return new Circle(); } always returns Circle, no flexibility. function createShape(type: string): Shape { if(type === 'circle') return new Circle(); else return new Square(); } uses string checks, less type-safe and harder to extend. function createShape<T>(): T { return {} as T; } returns an empty object cast to T, which is unsafe.

3. Final Answer:

   function createShape<T extends Shape>(ctor: new () => T): T { return new ctor(); } -> Option C

4. Quick Check:

   Generic factory with constructor param = flexible shape creation [OK]

Hint: Use generic constructor param for flexible object creation [OK]

Common Mistakes:

• Hardcoding return types limits extensibility
• Using string type checks reduces type safety
• Casting empty objects loses real behavior