Builder pattern with generics in Typescript - Mini Project: Build & Apply

Builder pattern with generics

📖 Scenario: You are creating a system to build different types of vehicles. Each vehicle has some common parts like wheels and color, but also some unique parts. You want to use a builder pattern with generics to create vehicles flexibly.

🎯 Goal: Build a generic VehicleBuilder class that can build vehicles with different specific parts. Then create a Car type and use the builder to create a car object.

📋 What You'll Learn

Create a generic VehicleBuilder<T> class with methods to set wheels and color.

Add a method setPart(key: keyof T, value: T[keyof T]) to set specific parts.

Add a build() method that returns the complete vehicle object combining common and specific parts.

Create a Car interface with engine and doors properties.

Use VehicleBuilder<Car> to build a car with 4 wheels, color 'red', engine 'V8', and 4 doors.

Print the final car object.

💡 Why This Matters

🌍 Real World

Builder patterns with generics help create flexible and reusable code for building complex objects like vehicles, UI components, or configurations.

💼 Career

Understanding builder patterns and generics is useful for software developers working on scalable TypeScript applications, improving code maintainability and readability.

1

Create the generic VehicleBuilder class

Create a generic class called VehicleBuilder<T> with a private property vehicle initialized as an empty object of type T & { wheels?: number; color?: string }.

Typescript

class VehicleBuilder {
    private vehicle: T & { wheels?: number; color?: string } = {} as T & { wheels?: number; color?: string };
    // Your code here
}

Hint

Use class VehicleBuilder<T> and initialize vehicle as an empty object with type T & { wheels?: number; color?: string }.

2

Add methods to set wheels, color, and specific parts

Add three methods inside VehicleBuilder<T>: setWheels(wheels: number): this, setColor(color: string): this, and setPart(key: keyof T, value: T[keyof T]): this. Each method should set the corresponding property on this.vehicle and return this.

Typescript

class VehicleBuilder {
    private vehicle: T & { wheels?: number; color?: string } = {} as T & { wheels?: number; color?: string };

    // Add setWheels, setColor, and setPart methods here
}

Hint

Each method sets a property on this.vehicle and returns this to allow chaining.

3

Add the build method and create Car interface

Add a method build(): T & { wheels?: number; color?: string } that returns this.vehicle. Then create an interface Car with properties engine: string and doors: number.

Typescript

class VehicleBuilder {
    private vehicle: T & { wheels?: number; color?: string } = {} as T & { wheels?: number; color?: string };

    setWheels(wheels: number): this {
        this.vehicle.wheels = wheels;
        return this;
    }

    setColor(color: string): this {
        this.vehicle.color = color;
        return this;
    }

    setPart(key: keyof T, value: T[keyof T]): this {
        this.vehicle[key] = value;
        return this;
    }

    // Add build method here
}

// Create Car interface here

Hint

The build method returns the complete vehicle object. The Car interface defines specific parts.

4

Use VehicleBuilder to build a Car and print it

Create a variable carBuilder as new VehicleBuilder<Car>(). Use method chaining to set wheels to 4, color to 'red', engine to 'V8', and doors to 4. Then create a variable car by calling carBuilder.build(). Finally, print car using console.log(car).

Typescript

class VehicleBuilder {
    private vehicle: T & { wheels?: number; color?: string } = {} as T & { wheels?: number; color?: string };

    setWheels(wheels: number): this {
        this.vehicle.wheels = wheels;
        return this;
    }

    setColor(color: string): this {
        this.vehicle.color = color;
        return this;
    }

    setPart(key: keyof T, value: T[keyof T]): this {
        this.vehicle[key] = value;
        return this;
    }

    build(): T & { wheels?: number; color?: string } {
        return this.vehicle;
    }
}

interface Car {
    engine: string;
    doors: number;
}

// Create carBuilder, build car, and print it below

Hint

Use method chaining on carBuilder to set all properties, then call build() and print the result.

Practice

1.

What is the main advantage of using the builder pattern with generics in TypeScript?

easy

A. It automatically generates UI components from objects.

B. It makes the code run faster by skipping type checks.

C. It allows creating different object types step-by-step with type safety.

D. It replaces all functions with classes.

Solution

1. Step 1: Understand builder pattern purpose

   The builder pattern helps create complex objects step-by-step, improving clarity and flexibility.

2. Step 2: Role of generics in TypeScript

   Generics allow the builder to work with any object type while keeping type safety, ensuring keys and values match.

3. Final Answer:

   It allows creating different object types step-by-step with type safety. -> Option C

4. Quick Check:

   Builder + generics = flexible, safe object creation [OK]

Hint: Generics + builder = reusable and type-safe object creation [OK]

Common Mistakes:

• Thinking builder auto-generates UI
• Believing it skips type checks
• Confusing builder with replacing functions

2.

Which of the following is the correct syntax to declare a generic builder class in TypeScript?

class Builder<T> {
  private obj: T;
  constructor() {
    this.obj = {} as T;
  }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T {
    return this.obj;
  }
}

easy

A. class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } }

B. class Builder<T> { private obj: T; constructor() { this.obj = new T(); } }

C. class Builder { private obj: T; constructor() { this.obj = {} as T; } }

D. class Builder<T> { private obj: any; constructor() { this.obj = {}; } }

Solution

1. Step 1: Check generic class declaration

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } correctly declares a generic class with <T> and initializes obj as {} as T.

2. Step 2: Identify incorrect initializations

   class Builder<T> { private obj: T; constructor() { this.obj = new T(); } } tries new T() which is invalid in TypeScript. class Builder { private obj: T; constructor() { this.obj = {} as T; } } misses generic <T>. class Builder<T> { private obj: any; constructor() { this.obj = {}; } } uses any losing type safety.

3. Final Answer:

   class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } -> Option A

4. Quick Check:

   Generic class with {} as T init = class Builder<T> { private obj: T; constructor() { this.obj = {} as T; } } [OK]

Hint: Use {} as T to initialize generic object safely [OK]

Common Mistakes:

• Trying to instantiate generic type with new
• Forgetting to declare generic parameter <T>
• Using any instead of generic type

3.

What will be the output of this code?

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {} as T; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

interface Person { name: string; age: number; }
const person = new Builder<Person>()
  .set('name', 'Alice')
  .set('age', 30)
  .build();
console.log(person);

medium

A. { name: 'Alice', age: 30 }

B. { name: 'Alice' }

C. Error: Cannot assign to 'name' of type 'T'

D. {}

Solution

1. Step 1: Understand builder usage

   The builder sets 'name' to 'Alice' and 'age' to 30 on an empty object typed as Person.

2. Step 2: Check final build output

   The build() method returns the object with both properties set, so output is { name: 'Alice', age: 30 }.

3. Final Answer:

   { name: 'Alice', age: 30 } -> Option A

4. Quick Check:

   Builder sets keys correctly = { name: 'Alice', age: 30 } [OK]

Hint: Builder sets keys stepwise, final object has all properties [OK]

Common Mistakes:

• Expecting partial object missing keys
• Thinking assignment causes error
• Assuming empty object returned

4.

Identify the error in this builder class and select the fix:

class Builder<T> {
  private obj: T;
  constructor() { this.obj = {}; }
  set(key: keyof T, value: T[keyof T]): this {
    this.obj[key] = value;
    return this;
  }
  build(): T { return this.obj; }
}

medium

A. Use any type for obj instead of T.

B. Change set method to accept key: string.

C. Remove generic <T> from class declaration.

D. Initialize obj as {} as T in constructor.

Solution

1. Step 1: Identify initialization error

   Assigning {} directly to obj: T causes a type error because {} is not guaranteed to be T.

2. Step 2: Correct initialization with type assertion

   Using {} as T tells TypeScript to treat empty object as type T safely during construction.

3. Final Answer:

   Initialize obj as {} as T in constructor. -> Option D

4. Quick Check:

   Use {} as T to fix generic object init [OK]

Hint: Use type assertion {} as T to initialize generic objects [OK]

Common Mistakes:

• Assigning {} directly without assertion
• Changing key type to string loses type safety
• Removing generics breaks flexibility

5.

You want to build a generic builder that only allows setting properties defined in the object type and prevents setting undefined keys. Which TypeScript feature helps enforce this at compile time?

hard

A. Using any type for keys to allow flexibility.

B. Using keyof T as the key type in the set method.

C. Using unknown type for values to accept any value.

D. Using Partial<T> for the object storage.

Solution

1. Step 1: Understand key restriction need

   To prevent setting keys not in the object type, the key parameter must be limited to valid keys.

2. Step 2: Use keyof T for key type

   keyof T represents all valid keys of type T, so using it restricts keys to only those defined in T.

3. Final Answer:

   Using keyof T as the key type in the set method. -> Option B

4. Quick Check:

   keyof T restricts keys to valid properties [OK]

Hint: Use keyof T to limit keys to object properties [OK]

Common Mistakes:

• Using any loses key restriction
• Using unknown does not restrict keys
• Partial<T> affects values, not keys