Middlewares
Problem statement
Implement a middlewares function that takes any number of middleware functions and composes them into a single callable function.
The composed function accepts a shared context, returns a Promise, and calls each middleware in order. The behavior should be similar to Koa middleware.
Middleware Shape
async function middleware(context, next) {
// read or mutate context
await next();
}Rules
- Each middleware receives
contextandnext. - Calling
next()invokes the next middleware. - If a middleware does not call
next(), the chain stops. - Execution must be asynchronous and sequential.
- Calling
next()more than once should reject with an error.
Requirements & constraints
- Return a Promise from the composed function.
- Support async and sync middlewares.
- Reject if `next()` is called more than once by the same chain step.
How to approach Middlewares
The strategy an interviewer expects you to reach for.
Middleware composition is Koa/Express style: each layer gets (context, next) and chooses when to continue the chain. Naive move Call every middleware in a for-loop without a next callback. You lose before/after semantics and short-circuiting. Where it breaks Calling next() twice must error. Falling off the end should resolve. Async middleware must be thenable-aware. The fix Dispatch by index. Guard i <= index to detect double next. next calls dispatch(i+1). Wrap return values in Promise.resolve. Remember: next is a single-step forward cursor — double-call is a hard error. Say this in the interview: I'd compose middleware with an index dispatcher, reject if next is called twice, and let each layer await next() for downstream work.
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