Generic factory pattern in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a generic factory function that returns an instance of type T.

Typescript

function createInstance<T>(ctor: { new(): [1] }): T {
  return new ctor();
}

Drag options to blanks, or click blank then click option'

Aobject

Bvoid

Cany

DT

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed type like 'object' instead of the generic type T.

Omitting the return type in the constructor signature.

The generic type T is used to specify the return type of the factory function. The constructor signature must return T to create an instance of that type.

2fill in blank

medium

Complete the code to define a generic factory interface with a create method.

Typescript

interface Factory<T> {
  create(): [1];
}

Drag options to blanks, or click blank then click option'

Avoid

Bany

CT

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as the return type which means no value is returned.

Using any which loses type safety.

The create method should return an instance of the generic type T.

3fill in blank

hard

Fix the error in the generic factory function signature to correctly accept a constructor.

Typescript

function factory<T>([1]: { new(): T }): T {
  return new ctor();
}

Drag options to blanks, or click blank then click option'

Actor

Bcreate

Cinstance

Dobj

Attempts:

3 left

💡 Hint

Common Mistakes

Using a parameter name different from the one used inside the function body.

Omitting the parameter name.

The parameter name ctor is used to represent the constructor function. It must match the name used inside the function body.

4fill in blank

hard

Fill both blanks to create a generic factory function that accepts a constructor and returns an instance.

Typescript

function create[1]<T>([2]: { new(): T }): T {
  return new ctor();
}

Drag options to blanks, or click blank then click option'

AInstance

BFactory

Cctor

Dobj

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent parameter names.

Using a function name that does not describe its purpose.

The function name createFactory describes the factory pattern. The parameter ctor is the constructor used to create the instance.

5fill in blank

hard

Fill all three blanks to implement a generic factory class with a create method.

Typescript

class Generic[1]<T> {
  constructor(private [2]: { new(): T }) {}
  create(): [3] {
    return new this.ctor();
  }
}

Drag options to blanks, or click blank then click option'

AFactory

Bctor

CT

DInstance

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class or parameter names.

Not using the generic type T consistently.

The class is named GenericFactory. The constructor parameter ctor is the constructor function. The create method returns an instance of type T.

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