Builder Pattern

Construct complex objects step by step:

interface User {
  name: string;
  email: string;
  age?: number;
  role?: string;
}

class UserBuilder {
  private user: Partial = {};

  setName(name: string): this {
    this.user.name = name;
    return this;
  }

  setEmail(email: string): this {
    this.user.email = email;
    return this;
  }

  setAge(age: number): this {
    this.user.age = age;
    return this;
  }

  setRole(role: string): this {
    this.user.role = role;
    return this;
  }

  build(): User {
    if (!this.user.name || !this.user.email) {
      throw new Error("Name and email are required");
    }
    return this.user as User;
  }
}

// Usage - fluent API
const user = new UserBuilder()
  .setName("Alice")
  .setEmail("alice@example.com")
  .setAge(30)
  .build();

Type-Safe Builder

// Require certain steps before build
interface RequiredFields {
  name: string;
  email: string;
}

class StrictUserBuilder = {}> {
  private user: Partial = {};

  setName(name: string): StrictUserBuilder {
    this.user.name = name;
    return this as StrictUserBuilder;
  }

  setEmail(email: string): StrictUserBuilder {
    this.user.email = email;
    return this as StrictUserBuilder;
  }

  setAge(age: number): StrictUserBuilder {
    this.user.age = age;
    return this as StrictUserBuilder;
  }

  setRole(role: string): StrictUserBuilder {
    this.user.role = role;
    return this as StrictUserBuilder;
  }

  build(this: StrictUserBuilder<{ name: string; email: string }>): User {
    return this.user as User;
  }
}