why module.exports = { router }; is throwing error while module.exports = router ; is not?

Question

C:\Users\RAHUL\Desktop\2.0 Developer course\YelpCamp\v1\node_modules\express\lib\application.js:210
    throw new TypeError('app.use() requires a middleware function')
    ^

TypeError: app.use() requires a middleware function
    at Function.use (C:\Users\RAHUL\Desktop\2.0 Developer course\YelpCamp\v1\node_modules\express\lib\application.js:210:11)
    at Object.<anonymous> (C:\Users\RAHUL\Desktop\2.0 Developer course\YelpCamp\v1\app.js:52:5)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12)
    at internal/main/run_main_module.js:18:47

Answer 1

It would be better to include your original code in the question and not just the error, but:

module.exports = router; means the export is the router object itself, which Express will be able to use as middleware if it's a valid router.

However, module.exports = { router }; exports an anonymous object with one key, called router, which points to your router object. It's equivalent to module.exports = { router: router };.

This is an example of the object shorthand notation, see this article from Mozilla.