What is the reason behind some middleware functions being passed in with invocation parentheses, while an anonymous function is passed in without being invoked?
app.use(logger());
app.use(bodyParser());
If logger() is evaluated immediately and the return value gets passed into app.use(), why doesn't app.use() receive undefined as the parameter?
Is it true that the function
logger()is immediately evaluated and its return value passed intoapp.use()?
Absolutely.
Why doesn't
app.use()receiveundefinedas the parameter?
The reason for this is that both logger() and bodyParser() are functions that actually return middleware, rather than being the middleware handlers themselves. This is a common practice in Express middleware: exporting a function that can accept options to configure the returned middleware.
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