异常调用以表达get方法。 应该错了，但是运行良好

Question

Express' get method has only two kinds of calls documented, however in passportjs.org's documentation it is shown being called in a third kind.

I am learning to implement SSO using google's Oauth 2.0 strategy. During this I came across an unusual usage of app.get method. Such a function call is not defined in express' documentation.

I referred to the following two pages, to ensure I wasn't mistaken:

1. Express JS documentation for app.get method
2. Passport JS documentation for app.get method

Express js documentation has only two kinds of calls:

1. app.get(name)
2. app.get(path, callback [, callback ...])

Now, the call in passportjs.org appears to be using the second form, but notice that second argument (passport.authenticate) is actually a function call, and not a function definition (ie callback) as it should be according to #2 above:

app.get('/auth/google',
  passport.authenticate('google', { scope: 'https://www.google.com/m8/feeds' });

I expect a run time error as the call to app.get, doesn't match any of the documented function call kinds. However, the function call runs well and completes the intended task. How?

Answer 1

From Passport npm page

Passport is Express-compatible authentication middleware for Node.js

The way this works is passport exposes methods which is compatible with callback that express expects.

The simplest callback signature which express expects is:

(req, res) => { /* something */ }

So any function which returns another function of the expected signature can be passed as middleware.

For example

const app = express()

function myFunc(some) {
  return (req, res) => {
    res.json(some);
  }
}

app.get('/', myFunc('something'));

*Express documentation also illustrates similar middleware function under Configurable middleware in the link above