Different way of calling a function in javascript

Question

Hey I was learning Node as part of course where they made some variable equal to a function

Something like this

const authCheck = (req, res, next) => {
    if (req.user) {
        next()
    } else {
        res.redirect("http://localhost:3000/")
    }

}

and then they called it in a middleware

router.post("/", authCheck, (req, res) =>  {

Now, I have usually seen people calling a function like this authCheck() While that works, I wanted to comprehend the difference between both of them

authCheck() would call the function, (with no arguments, by the way) ... authCheck passes the function as an argument to router.post in your example - and something in router.post may call authCheck if required — Bravo
– Bravo, Commented Oct 30, 2018 at 8:28
Now, I have usually seen people calling a function like this authCheck() - consider providing the example where you saw this. This would be incorrect and result in error. — Estus Flask
– Estus Flask, Commented Oct 30, 2018 at 8:39

Shilly · Accepted Answer · 2018-10-30 08:29:45Z

1

router.post("/", authCheck, (req, res) => { This line does not actually call the function. It passed the function as a variable into the router.post() function.

That router function will then in time call authCheck, once a request arrives at the router function.

So you're correct that functions gets called by writing () behind the function name. Remember that functions are so called 'first-class', meaning they can be used as parameters, passed around, anything you can do with a variable.

If you would write router.post("/", authCheck(), (req, res) => {, instead of passing the authCheck function into the router, you would immediately call authCheck and pass the return of authCheck into router.post() instead.

answered Oct 30, 2018 at 8:29

Shilly

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3 Comments

Alwaysblue Over a year ago

Shilly firstly thanks a lot for answering, just to summarise, if we pass say some function as a parameter function something (authCheck(), somethingElse), then return of authCheck() will be the return of something and somethingElse would never been executed. Hence whenever we are passing function as a parameter we should always use 'authCheck`?

Shilly Over a year ago

Euh? I don't get what you mean. Any function can be passed. authCheck is just the function that gets passed in this example. If you have something( authCheck(), somethingElse ), then the function something will be called. It takes two parameters. The second parameter is the variable somethingElse. The first parameter will be whatever authCheck returns. I expect authCheck to return like an object containing user credentials.

Shilly Over a year ago

That returned object will then be the first parameters used inside something(). If somethingElse is also a function, it will depend completely on how something() is written to determine when to either use the object returned from calling authCheck() at the start, or when to call somethingElse(). The clue of everything is, any function only gets called once they have the () behind them.

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