Lesson 23 — Middleware compose

A chain of handlers

In Express and Koa, a request passes through middleware: small functions that each do one job (log, check login, parse JSON) and then call next() to hand over to the next one. Code after next() runs on the way back, like layers of an onion.

compose turns a list of middleware into one function. dispatch(i) runs middleware number i and gives it a next that calls dispatch(i + 1).

Note: this playground runs JavaScript in your browser, not in real Node, so Node-only tools (like fs or http) are not available. The exercise is a small plain-JS simulation of the Node idea.

Why it matters: this is how almost every Node web framework structures a request.