Compression middleware order in express

Question

Express API docs for app.use() say:

Middleware functions are executed sequentially, therefore the order of middleware inclusion is important.

Then, I see code like this all over the place:

import compression from 'compression';
import api from './api';
// some code 
app.use(compression() );
// some more code 
app.use(api);

Where api is the router which does res.send() calls.

As I see, compression middleware is registered before sending respose. Then how does it compress response? Is there some kind of hook to register callback after res.send() call?

Answer 1

The compression middleware is executed sequentially, but it's overloading the res.write res.on and res.end functions. So when the api will call those later, it wil run the compression code.

ref : https://github.com/expressjs/compression/blob/93586e75a0a1c5bbfd353c4cec1cfcee2e52adde/index.js#L78