Source code for https://qiita.com/drafts/69341387ed6a493b5df5

builder-pattern.ts

// Builderオブジェクトの型
type Builder<Props, Result> = ({} extends Props
  ? {
      build: () => Result;
    }
  : {}) &
  { [P in keyof Props]-?: SetFunction<Props, P, Result> };

type SetFunction<Props, K extends keyof Props, Result> = (
  value: Exclude<Props[K], undefined>
) => Builder<Pick<Props, Exclude<keyof Props, K>>, Result>;

type BuildFunction<Props, Result> = (props: Props) => Result;

const propsObject = Symbol();
const buildFunction = Symbol();
class BuilderImpl<Props, Result> {
  constructor(bf: BuildFunction<Props, Result>) {
    return new Proxy(
      {
        [propsObject]: {},
        [buildFunction]: bf
      },
      {
        get(target: any, prop: any, receiver: any) {
          if (prop === "build") {
            // build関数
            return () => target[buildFunction](target[propsObject]);
          } else {
            // それ以外はsetter関数
            return (value: any) => {
              target[propsObject][prop] = value;
              return receiver;
            };
          }
        }
      }
    );
  }
}

function builderFactory<Props, Result>(
  bf: BuildFunction<Props, Result>
): new () => Builder<Props, Result> {
  return class {
    constructor() {
      return new BuilderImpl(bf);
    }
  } as any;
}

const FooBarBuilder = builderFactory<
  {
    foo: number;
    bar: string;
  },
  string
>(({ foo, bar }) => `foo = ${foo}, bar = ${bar}`);

const foobarValue = new FooBarBuilder()
  .foo(123)
  .bar("456")
  .build();
console.log(foobarValue); // foo = 123, bar = 456

const foobarValue2 = new FooBarBuilder()
  .bar("bar")
  .foo(0)
  .build();
console.log(foobarValue2); // foo = 0, bar = bar

(new FooBarBuilder).build();
//                  ^^^^^
// エラー: Property 'build' does not exist on type '...'
(new FooBarBuilder).foo(456).build();
//                           ^^^^^
// エラー: Property 'build' does not exist on type '...'
(new FooBarBuilder).foo(456).foo(123);
//                           ^^^
// エラー: Property 'foo' does not exist on type '...'

(new FooBarBuilder).foo(123).bar("bar").build()

const FooBarBazBuilder = builderFactory<
  {
    foo: number;
    bar: string;
    baz?: string;
  },
  string
>(({ foo, bar, baz }) => {
  if (baz != null) {
    return `foo = ${foo}, bar = ${bar}, baz = ${baz}`;
  } else {
    return `foo = ${foo}, bar = ${bar}`;
  }
});

const foobarbazValue = new FooBarBazBuilder()
  .foo(123)
  .bar("bar")
  .baz("bazbaz")
  .build();
console.log(foobarbazValue); // foo = 123, bar = bar, baz = bazbaz

const foobarbazValue2 = new FooBarBazBuilder()
  .bar("bar")
  .foo(0)
  .build();
console.log(foobarbazValue2); // foo = 0, bar = bar

const foobarbazValue3 = new FooBarBazBuilder()
  .baz("baz")
  .bar("123")
  .foo(444)
  .build();
console.log(foobarbazValue3);