Optional callback not being called in the node.js async module's forEachOf method

问题

I'm using the async module's forEachOf method to print the end result after iterating through an object. Here is a shortened version of what I'm doing:

var async = require('async'),
    cheerio = require('cheerio'),
    request = require('request');

var returnArray = [];

async.forEachOf(myObj, function (value, key, callback) {
    var anotherObj = {};

    anotherObj.type = "val1";

    request(someurl, function(err, res, body) {
        if (err) {
            return callback(err);
        }

        var $ = cheerio.load(body);

        anotherObj.name = "val2";

        var miniObj = {};
        $('#some-element', "#an-id").each(function(i, value) {
            var val = value.value;
            miniObj[size] = val;
        });

        anotherObj.miniObj = miniObj;

        returnArray.push(anotherObj);

        return callback();
    });
}, function (err) {
    if (err) {
        console.error(err.message);
    }

    console.log(returnArray);
});

However, when I run the program, nothing (namely, 'returnArray') gets printed to the console like it should be.

For reference, I have looked at these other, similar posts:

Using async module to fire a callback once all files are read (seems outdated, is using the wrong method, and doesn't address the issue)
Last callback not being called using async (doesn't address the issue)

I'm not sure what I'm doing wrong here. Could anyone please point out what I'm doing wrong?

Thanks!

EDIT: So I think I finally figured out what I was doing wrong. In a different example I provided earlier HERE, using Runnable I forgot to add a 'return callback()' statement. Then it worked. The only difference between that and this example being that my 'return callback()' statement in the above-provided example is itself called within another asynchronous method. I think in order to fix my problem I will somehow have to make sure (probably using some control flow function in async's library) to call 'return callback()' at the correct 'scope/level' after the second asynchronous method has finished. I think I will attribute this, my mistake, to the lack of documentation on proper usage of the 'return callback()' statement in the async docs. I will update this post with a solution once I figure it out (and catch up on some sleep)!

回答1:

Your statement:

if (err) {
    return callback(err);
}

is not valid for asynchronous programming. Instead, you should do:

if(err) callback(err);

This is why you aren't getting anything returned. I rewrote your code with async concepts applied:

var async = require('async'),
var cheerio = require('cheerio'),
var request = require('request');

var returnArray = [];
async.forEachOf(myObj, function (value, key, next) {
    var anotherObj = {};

    anotherObj.type = "val1";

    request(someurl, function(err, res, body) {
        if (err) next(err);

        var $ = cheerio.load(body);

        anotherObj.name = "val2";

        var miniObj = {};
        async.each($('#some-element', "#an-id"), function (value, next) {
          var val = value.value;
          miniObj[size] = val;
        });

        anotherObj.miniObj = miniObj;

        returnArray.push(anotherObj);

        next();
    });
}, function (err) {
    if (err) console.error(err.message);

    console.log(returnArray);
    callback(returnArray);

});

Notice that you have two different named callbacks. The outer function callback is called callback. The inner function callback is called next.

来源：https://stackoverflow.com/questions/31097142/optional-callback-not-being-called-in-the-node-js-async-modules-foreachof-metho

标签

javascript

node.js

asynchronous

callback

async.js