Why do I get 'Next is not a function' when I try to export a Middleware function to module.exports object?

Question

I have a middleware function written inside my logger.js module which I then import into app.js and use

// ------ File : logger.js ------ //

function log(req, res, next) {
  console.log('Logging details ... ');
  next();
}

module.exports = log;

// ------ File : app.js -------- //

const logger = require('./logger');

app.use(logger);

The above code works without any issue and my log functionality works. However, if I export this log function in the following way (add it to the module.exports object), I get an error

// ------ File : logger.js -------//

function log(req, res, next) {
  console.log('Logging details ... ');
  next();
}
module.exports.log = log;

// ------ File : app.js -------- //

const logger = require('./logger');

app.use(logger.log());

Logging details ...
D:\express-demo-worked\logger.js:4
    next();
    ^

TypeError: next is not a function
    at Object.log (D:\express-demo-worked\logger.js:4:5)
    at Object.<anonymous> (D:\express-demo-worked\app.js:18:16)
    at Module._compile (internal/modules/cjs/loader.js:738:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:749:10)
    at Module.load (internal/modules/cjs/loader.js:630:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:570:12)
    at Function.Module._load (internal/modules/cjs/loader.js:562:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:801:12)
    at internal/main/run_main_module.js:21:11
[nodemon] app crashed - waiting for file changes before starting...

Can someone explain to me why this behaves differently and how to correct the second code snippet I added here?

I'm no expert with express, or the ES6 module system - but it appears to me that you've made a simple mistake of making the function call in `app.use(logger.log());` rather than simply passing the function reference: `app.use(logger.log);` — Robin Zigmond, Mar 15 '19 at 15:28
Possible duplicate of [*setTimeout calls function immediately instead of after delay*](https://stackoverflow.com/questions/27643714/settimeout-calls-function-immediately-instead-of-after-delay)? — T.J. Crowder, Mar 15 '19 at 15:32

Nicholas Tower · Accepted Answer · 2019-03-15T15:36:22.517

3

app.use(logger.log());

This will call logger.log immediately, passing in no arguments. Whatever logger.log returns will then be passed into app.use. Since you passed in no arguments, next is undefined, resulting in that exception.

Instead, do:

app.use(logger.log);

This will pass logger.log into app.use. At some point later, logger.log will be called, passing in the correct arguments.

edited Mar 15 '19 at 15:36

answered Mar 15 '19 at 15:28

Nicholas Tower

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score 2 · Answer 2 · answered Mar 15 '19 at 15:29

Here:

app.use(logger.log());

you're calling log, without any arguments, and passing its return value into app.use. Since log expects and uses its parameters, that will fail on next() because the value of the next parameter is undefined, as you didn't pass an argument for it.

You may have meant just to pass the function in:

app.use(logger.log);
// No () ---------^

Why do I get 'Next is not a function' when I try to export a Middleware function to module.exports object?

2 Answers2