Curried Default Error Handler Failing

[JesterXL]

It appears that Express checks the function's arity to identify if it's an error-handler route. It works with a function, and works with an arrow function, but does not work with a Lodash curried function. I believe this is possibly because theFunction.length will print out 0 for curried functions, whereas function and ()=> will print out a true arity. However, I can't seem to find where in the code y'all are checking for this. I've tried to manually setting the theFunction.length to 3 or 4 to hack it, but no dice. Any pointers?

Works when you hit localhost:3000/error:

const express = require('express')
const curry = require('lodash/fp/curry')
const app = express()

app.get('/', (req, res) => res.send('Hello World!'))
app.get('/error', (req, res, next) => {
    next(new Error('boom'))
})

function genericErrorHandler(err, req, res, next) {
    console.log("err message:", err.message)
    res.status(500).send('Something broke!');
}
app.use(genericErrorHandler)

app.listen(3000, () => console.log('Example app listening on port 3000!'))

Doesn't work:

const express = require('express')
const curry = require('lodash/fp/curry')
const app = express()

app.get('/', (req, res) => res.send('Hello World!'))
app.get('/error', (req, res, next) => {
    next(new Error('boom'))
})

const genericErrorHandler = curry((prefix, err, req, res, next) => {
    console.log(`${prefix} err message:`, err.message)
    res.status(500).send('Something broke!');
})
const curried = genericErrorHandler('🐮')
curried.length = 4 // I tried, heh
app.use(curried);

app.listen(3000, () => console.log('Example app listening on port 3000!'))

[dougwilson]

You're correct, this is the documented way we determine what is a middleware and what is an error middleware. I don't think you can set the length of a function in javascript after the fact; it has to have the arguments at the time of declaration.

[dougwilson]

For example:

const genericErrorHandler = (prefix) => (err, req, res, next) => {
    console.log(`${prefix} err message:`, err.message)
    res.status(500).send('Something broke!');
}

[wesleytodd]

I have this open, and plan on switching it to the implementation at the bottom soon. Would love input.

pillarjs/router#59

[JesterXL]

Got it, thanks. I'll just create a wrapper function with that value fixed.

[JesterXL]

BTW, those stumbling here, I found way to make this work while still allowing a longer arity for easier unit testing & acting like a curried function using default parameters.

Take the curried function:

const genericErrorHandler = curry((prefix, err, req, res, next) => {
    console.log(`${prefix} err message:`, err.message)
    res.status(500).send('Something broke!');
})

And just build a wrapper (or ditch the curried one), and move the default curried left parameters to the end with defaults:

const genericErrorHandler = (err, req, res, next, prefix='some value') => {
    console.log(`${prefix} err message:`, err.message)
    res.status(500).send('Something broke!');
})

In my case, my prefix was a function; both work, the arity is always 4:

const specialLog = ()=> {...}
const genericErrorHandler = (err, req, res, next, specialLogFunction=specialLog) => {
   ... 
   specialLogFunction(err)
   ...
})

[wesleytodd]

Also, if anyone is interested, you can do this with a simple middleware factory function:

const genericErrorHandler = (prefix) => {
  return (err, req, res, next) => {
    console.log(`${prefix} err message:`, err.message)
    res.status(500).send('Something broke!')
  }
}
app.use(genericErrorHandler('foo'))