async.each-回调函数是什么？

Question

I just want to check a list of urls to see if each exists and continue after all have been done:

var urls = [ "http://...", "http://...", ... ];

async.each(urls, function(url, ??callback??) {
             http.get(url, function(response) {
                 console.log(response.statusCode);
             });
           }, function(err) {
                if (!err)
                   console.log("All urls called");
           });

What do I put for ??callback?? when I have nothing to continue after each task is completed? The documentation doesn't indicate it is optional.

Answer 1

The second parameter is iteratee , which is an AsyncFunction :

AsyncFunction()

An "async function" in the context of Async is an asynchronous function with a variable number of parameters, with the final parameter being a callback. ( function (arg1, arg2, ..., callback) {} ) The final callback is of the form callback(err, results...) , which must be called once the function is completed. The callback should be called with a Error as its first argument to signal that an error occurred. Otherwise, if no error occurred, it should be called with null as the first argument, and any additional result arguments that may apply, to signal successful completion. The callback must be called exactly once, ideally on a later tick of the JavaScript event loop.

It's an argument async calls your iteratee with. In your case, you need to call it when the get is complete (successful or failed):

var urls = [ "http://...", "http://...", ... ];
async.each(urls, function(url, callback) {
    http.get(url, function(response) {
        console.log(response.statusCode);
        callback(null); // <====
    });
}, function(err) {
    if (!err)
        console.log("All urls called");
});

async needs to know the operation has completed so it can manage the overall process.

My example above is simplistic, you may want to differentiate success from failure, but the idea is that you must call the callback you receive, either with an error (first arguments) or success (first argument = null , optional second argument).

Answer 2

You're meant to call the callback when you've completed each async task, ie getting the URL.

So something like...

http.get(url, function(response) {
  // Here you call it with `null` to signify a non-error completion.
  callback(null);
});

...inside of your main each function. If all you wanted to do was call it without any further code, you could use http.get(url, callback.bind(null, null)) .