How do you implement a type-safe builder pattern?

Type-safe builder pattern uses method chaining with conditional types to track which properties have been set. Basic Builder Pattern: interface User { id: string; name: string; email: string; age?: number; } type RequiredKeys = { [K in keyof T]-?: {} extends Pick ? never : K; }[keyof T]; type Builder = { [P in RequiredKeys]: P extends K ? Builder : (value: T[P]) => Builder; } & (RequiredKeys extends K ? { build(): T } : {}); class UserBuilder implements Builder { private user: Partial = {}; id(value: string) { this.user.id = value; return this; } name(value: string) { this.user.name = value; return this; } email(value: string) { this.user.email = value; return this; } build(): User { return this.user as User; } } // Usage const user = new UserBuilder() .id('123') .name('John') .email('john@example.com') .build(); Advanced Generic Builder: class GenericBuilder { private data: Partial = {}; set(key: K, value: T[K]) { this.data[key] = value; return this; } build(): T { return this.data as T; } } Benefits: • Ensures required properties are set before building • Provides excellent IDE autocomplete • Prevents runtime errors from missing properties • Creates fluent, readable APIs.