JavaScript箭头功能

Question

I have seen some code that looks like this:

function printErr(err) {
    console.error(err)
}
request.on('error', err => printErr)

How does that work? I have spent hours trying to figure it out and searching everywhere for it. I know that printErr is an object with a function that is being returned but what is the mechanism that lets it put the err parameter into the function and call it?

I am thinking along the lines of:

function foo(callback){
    callback()
}

request.on('error', err => foo)

But how does it get the arguments?

Answer 1

This is a short hand for the arrow function. Whenever there is only one argument we can call it even without parentheses and => represents the return . When there is only a single line return we can explicitly use => for returning rather then using return.

 var a=e=>"hey"; console.log(a()) 

This is same as the above

 var a=(e)=>{return "hey"}; console.log(a()) 

Answer 2

I was confused at first but it was because the code I was analyzing was mistaken.

to simplify

function print(data) {
    console.log(data)
}

const foo = [1,2,3,4]

foo.forEach(x => print) // will not print and just return undefined.
foo.forEach(x => print(x)) // will print correctly