NodeJS / Express: what is “app.use”?

Question

In the docs for the NodeJS express module, the example code has app.use(...).

What is the use function and where is it defined?

Answer 1

app.use applies the specified middleware to the main app middleware stack. When attaching middleware to the main app stack, the order of attachment matters; if you attach middleware A before middleware B, middleware A will always execute first. You can specify a path for which a particular middleware is applicable. In the below example, “hello world” will always be logged before “happy holidays.”

const express = require('express')
const app = express()

app.use(function(req, res, next) {
  console.log('hello world')
  next()
})

app.use(function(req, res, next) {
  console.log('happy holidays')
  next()
})