Arguments object different to arguments

Question

I've hit a really strange issue, with Node & Express.

I have a middleware function, which can either be invoked in three different ways:

app.get('/', auth, handler)
app.get('/', auth('role'), handler)
app.get('/', auth('role', 'scope'), handler)

The obvious way to do this would be like this:

exports.auth = (a, b, c) => {

    let role, scope;

    switch(arguments.length) {
        case 3:
            // Called directly by express router
            handleAuth(a, b, c);
            break;

        case 2:
        case 1:
            // Called with role/scope, return handler
            // but don't execute
            role = a;
            scope = b;
            return handleAuth;
    }


    function handleAuth(req, res, next) {

        // Do some custom auth handling, can
        // check here if role/scope is set
        return next();

    }
}

However, I'm getting some very odd results for arguments.length. When called in the 2nd way, arguments.length == 5, and none of the arguments is role.

[ '0': '[object Object]',
  '1': 'function require(path) {\n    try {\n      exports.requireDepth += 1;\n      return mod.require(path);\n    } finally {\n      exports.requireDepth -= 1;\n    }\n  }',
  '2': '[object Object]',
  '3': '/Users/redacted/auth/index.js',
  '4': '/Users/redacted/auth' ]

If I log a, b, c within the function, I get a: 'role', b: undefined, c: undefined.

I've tried to reproduce it in Runkit, but haven't had any luck: https://runkit.com/benedictlewis/5a2a6538895ebe00124eb64e

Crogo · Accepted Answer · 2017-12-08 14:00:06Z

1

arguments is not exposed in arrow functions (() =>). If you need them, use a regular function instead.

exports.auth = function(a, b, c) {

  let role, scope;

  switch(arguments.length) {
    ...

Side note: The arguments your refer to in that arrow function is actually picked from the wrapper function which Node.js uses while running each module / required code. That function lets you access the "magical" variables such as exports, require, module, __dirname and __filename, which is why you see 5 arguments.

answered Dec 8, 2017 at 14:00

Crogo

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Comments

Aikon Mogwai · Accepted Answer · 2017-12-08 11:42:21Z

0

I think that case is broken.

'use strict';
let express = require('express');
let app = express();

//app.get('/', auth, handler);
app.get('/', auth('role'), handler);
//app.get('/', auth('role', 'scope'), handler);

function auth (a, b, c) {
    let role;   
    if (a == 'role') {
        role = 'user';
        return handleAuth;
    }   

    if (a != 'role')        
        return handleAuth(a, b, c);

    function handleAuth(req, res, next) {
        console.log(role || 'guest');
        next();  
    }
}

function handler (req, res, next) {
    res.send('OK');
}

app.listen(3000, () => console.log('Listening on port 3000'));

P.S. Imho, this code is knotty.

answered Dec 8, 2017 at 11:42

Aikon Mogwai

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