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JavaScript Build a REST API With Express Building API Routes in Express Using Middleware in Express

How does express determine the 'next' parameter to pass into each middleware function?

I'm curious as to how express knows what to pass as each next parameter. Is this simply done in order of app.use() functions, or is there more to it?


1 Answer

Tom Geraghty
Tom Geraghty
24,162 Points

The next parameter is a callback function. It's called next by convention to make writing and reading applications using this convention easier.

If you think in Promises, you can think of this particular callback as being called when the Promise has been resolved and the program is free to move on to the next step in processing the application's logic.

There's some useful info here: https://expressjs.com/en/guide/writing-middleware.html and here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise