TypeScript helpers for fluent types.

fluent-example.ts

import type { Fluent } from "./fluent";

export class User {
  public id: string;
  public username: string;

  private constructor() {
    this.id = "";
    this.username = "";
  }

  static create(): Fluent<User> {
    return new User();
  }

  withId(id: string): this {
    this.id = id;
    return this;
  }

  withUsername(username: string): this {
    this.username = username;
    return this;
  }
}

fluent.ts

export type Fluent<T, Keys extends keyof T = FluentKeys<T>> = {
  [K in Keys]: ChangeReturn<
    T[K],
    Exclude<Keys, K> extends never ? T : Fluent<T, Exclude<Keys, K>>
  >;
};

type FluentKeys<T> = {
  [K in keyof T]: T[K] extends (...args: any[]) => T ? K : never;
}[keyof T];

type ChangeReturn<F, Return> = F extends (...args: infer Args) => any
  ? (...args: Args) => Return
  : never;