Why Generic factory pattern in Typescript? - Purpose & Use Cases

The Big Idea

What if you could build any object with just one simple tool, no matter how many types you need?

The Scenario

Imagine you need to create many different types of objects in your program, like cars, bikes, and trucks. You write separate code for each type, repeating similar steps over and over.

The Problem

This manual way is slow and boring. If you want to add a new type, you must write new code from scratch. It's easy to make mistakes and hard to keep track of all the different creation steps.

The Solution

The generic factory pattern lets you write one smart piece of code that can create any type of object you want. You just tell it what type to make, and it handles the rest. This saves time and keeps your code clean and easy to change.

Before vs After

✗ Before

function createCar() { return new Car(); }
function createBike() { return new Bike(); }

✓ After

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

What It Enables

It makes your code flexible and ready to build many kinds of objects without rewriting creation logic each time.

Real Life Example

Think of a toy factory that can switch between making dolls, cars, or robots just by changing a setting, instead of building a new machine for each toy.

Key Takeaways

Manual object creation repeats code and wastes time.

Generic factory pattern creates any object type with one flexible function.

This approach makes adding new types easy and your code cleaner.

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