Middlewares

Requirements

• Export compose(middlewares) as named and default. Validate that the input is an array of functions, throwing TypeError synchronously for invalid entries. Capture the array without mutating it.
• Return an asynchronous runner (context, finalNext?) => Promise. Each middleware receives the same context and a next() function that runs the next middleware.
• Middleware chooses whether to call next. When it awaits next, code before next runs in forward order and code afterward unwinds in reverse order. Middleware must await or return its downstream promise.
• The runner resolves with the first middleware’s returned value, or undefined for an empty pipeline. finalNext, when supplied, is a zero-argument async function called after the last middleware. An empty pipeline can still call finalNext.
• Reject a second call to the same next function in one execution. Convert middleware throws to promise rejections and propagate downstream errors unless middleware catches them.
• Keep execution state separate for every run, including overlapping runs. Calling a saved next after its middleware has completed, cancellation, and automatic detection of an unawaited next are outside scope.

Examples

Input: const run = compose([
  async (ctx, next) => { ctx.push("before"); await next(); ctx.push("after"); },
  async (ctx) => { ctx.push("middle"); },
]);
const log = [];
await run(log);
Output: log is ["before", "middle", "after"].

Input: await compose([])({}, async () => "done");
Output: "done"