像 fs.writeFile() 回调参数这样的函数是如何工作的？

Question

I'm learning JavaScript at the moment, specifically Node JS. And during a basic tutorial I notice that some functions have callbacks with predefined arguments. For example

fs.writeFile(
path.join(__dirname, 'test', 'hello.txt'), 
"Hello World", 
err => {
if (err) throw err;
console.log('File Created...')})

I want to know where was "err" defined as a ErrNoException type.

Thanks!

Answer 1

I want to know where was "err" defined as a ErrNoException type.

In this type of interface, you pass a callback function and fs.writeFile() determines what arguments it will pass that callback when it calls it. It is up to you to declare your callback function to have appropriately named arguments that match what the caller will be passing it. So, YOU must declare the callback with an appropriately named argument. Since Javascript is not a typed language, you don't specific what the type of that argument is. The type of the argument is determined by the code that calls the callback. In this case that is the fs.writeFile() implementation. To use the callback appropriately, you have to read the documentation for fs.writeFile() so that you know how to use the argument to the callback.

So, in this specific case, fs.writeFile() requires a callback function and that callback function will be passed one argument and that argument will either by null (if there was no error) or will be an Error object that explains what the error was). You must learn this by either reading the code for fs.writeFile() or reading some documentation for it.

It's important to realize that when you define your callback function, all you are doing is providing a name for the arguments you expect the callback to be called with. You can then use that name in your code to reference those arguments. The actual data that is passed to that callback when it is called will be supplied by the fs.writeFile() function when it calls the callback.

It's unclear exactly what you mean by "defined as a ErrNoException type". Nodejs has an internal function (not available for external use) called errNoException() and you can see the source here . It uses that function internally for creating an Error object with some common properties such as an error code and a stack trace.

Answer 2

You can write similar functions yourself. Callback is simply a function (F) which is passed to another function(A) as a parameter. Function (A) executes some tasks and then executes Function F from parameters. For example:

function getData(url, callback) {
    fetch(url)
        .then(res => callback(null, res))
        .catch(err => callback(err, null))
}

In the sample function, what happens is, after making a fetch request, we wait until a response or error is generated. If a response is there, we simply pass that to callback param 2 and call first param to null and vice versa. Now, while calling the function,

getData('http://stackoverflow.com', (err, res) => {
     if(err)
         return console.log(err)
     console.log(res)
})