Variadic Tuple Inference for Type-Safe Middleware Pipelines

type Middleware<TContext> = (ctx: TContext, next: () => Promise<void>) => Promise<void>;
 
class Pipeline<TContext> {
  private middlewares: Middleware<TContext>[] = [];
 
  use(...fns: Middleware<TContext>[]): this {
    this.middlewares.push(...fns);
    return this;
  }
 
  async execute(ctx: TContext): Promise<void> {
    const runner = async (index: number): Promise<void> => {
      if (index >= this.middlewares.length) return;
      const mw = this.middlewares[index];
      await mw(ctx, () => runner(index + 1));
    };
    await runner(0);
  }
}
 
interface HttpContext { reqId: string; status?: number; }
const pipeline = new Pipeline<HttpContext>()
  .use(async (ctx, next) => {
    ctx.status = 200;
    await next();
  });