异步JS-Waterfall中的函数无法正确执行回调

Question

I am trying to get back to values from the function getFeed(), feedItems and feedMeta. I receive the following:

/Users/react-backend/node_modules/async/dist/async.js:228
    return supportsSymbol && fn[Symbol.toStringTag] === 'AsyncFunction';
                               ^

TypeError: Cannot read property 'Symbol(Symbol.toStringTag)' of undefined
    at isAsync (/Users/react-backend/node_modules/async/dist/async.js:228:32)
    at wrapAsync (/Users/react-backend/node_modules/async/dist/async.js:232:12)
    at /Users/react-backend/node_modules/async/dist/async.js:3866:9
    at replenish (/Users/react-backend/node_modules/async/dist/async.js:998:17)
    at /Users/react-backend/node_modules/async/dist/async.js:1002:9
    at eachOfLimit (/Users/react-backend/node_modules/async/dist/async.js:1027:24)
    at /Users/react-backend/node_modules/async/dist/async.js:1032:16
    at _parallel (/Users/react-backend/node_modules/async/dist/async.js:3865:5)
    at Object.series (/Users/react-backend/node_modules/async/dist/async.js:4721:5)
    at Object.<anonymous> (/Users/react-backend/feedParser2.js:114:7)
    at Module._compile (module.js:662:30)
    at Object.Module._extensions..js (module.js:673:10)
    at Module.load (module.js:575:32)
    at tryModuleLoad (module.js:515:12)
    at Function.Module._load (module.js:507:3)
    at Function.Module.runMain (module.js:703:10)

in the console when using the code immediately below:

function getFeed(callback) {
    let req = request(urlTestFeed);
    let feedparser = new FeedParser(feedParserOptions);
    let feedItems = [];
    let feedMeta = null;

    req.on('response', function(response) {
        let stream = this;
        if (response.statusCode == 200) {
            stream.pipe(feedparser);
        }
    });

    req.on('error', function(err) {
        console.log('getFeed: err.message == ' + err.message);
    });

    feedparser.on('meta', function() {
        try {
            feedMeta = this.meta;
        } catch (err) {
            console.log('getFeed: err.message == ' + err.message);
        }
    });

    feedparser.on('readable', function() {
        try {
            let item = this.read();
            if (item !== null) {
                feedItems.push(item);
            }
        } catch (err) {
            console.log('getFeed: err.message == ' + err.message);
        }
    });

    feedparser.on('end', function() {
        callback(undefined, feedItems, feedMeta);
    });

    feedparser.on('error', function(err) {
        console.log('getFeed: err.message == ' + err.message);
        callback(err);
    });
}

async:

async.waterfall([
        getFeed(function(err, feedItems, feedMeta) {
            console.log(feedMeta)
            if (!err) {
                if (feedMeta.title && feedMeta.description && feedMeta.link) {

                }
            }
        })]);

If I add callback(undefined, feedItems, feedMeta); to the last line in getFeed:

function getFeed(callback) {
    let req = request(urlTestFeed);
    let feedparser = new FeedParser(feedParserOptions);
    let feedItems = [];
    let feedMeta = null;

    req.on('response', function(response) {
        let stream = this;
        if (response.statusCode == 200) {
            stream.pipe(feedparser);
        }
    });

    req.on('error', function(err) {
        console.log('getFeed: err.message == ' + err.message);
    });

    feedparser.on('meta', function() {
        try {
            feedMeta = this.meta;
        } catch (err) {
            console.log('getFeed: err.message == ' + err.message);
        }
    });

    feedparser.on('readable', function() {
        try {
            let item = this.read();
            if (item !== null) {
                feedItems.push(item);
            }
        } catch (err) {
            console.log('getFeed: err.message == ' + err.message);
        }
    });

    feedparser.on('end', function() {
        callback(undefined, feedItems, feedMeta);
    });

    feedparser.on('error', function(err) {
        console.log('getFeed: err.message == ' + err.message);
        callback(err);
    });
    callback(undefined, feedItems, feedMeta);
}

I get the following in the console:

null
/Users/react-backend/feedParser2.js:119
            if (feedMeta.title && feedMeta.description && feedMeta.link) {
                         ^
TypeError: Cannot read property 'title' of null

What happened to the value of feedMeta when I added the callback? How can I pass the values from feedparser,feeditems, and, err to Async.waterfall?

Answer 1

Guessing this is too little, too late - but the problem isn't that getFeed isn't executing (it is actually executing - but you're returning before you get the results back from trying to read from a URL). The issue is that you're mixing asynchronous patterns with synchronous patterns.

I'd recommend simplifying what you're trying to do before moving this into async waterfall.

Specifically, gut the contents of your readFeed to be this:

function getFeed(callback){
    console.log('getFeed');
    return callback(null, [], {});
}

Then call it using the following

async.waterfall([
    function first (next){
        getFeed(function(err, feedItems, feedMeta) {
            console.log('getFeed cb:', feedMeta);
            if (!err) {
                if (feedMeta.title && feedMeta.description && feedMeta.link) {

                }
            }
            return next(err);
        })        
    }
],
function(err){
    console.log('waterfall done: err:',err);
});

Here is an example of the "flow of waterfall" (ie more than a single function being given to the waterfall):

async.waterfall([
    function first (next){
        getFeed(function(err, feedItems, feedMeta) {
            console.log('getFeed cb:', feedMeta);
            if (!err) {
                if (feedMeta.title && feedMeta.description && feedMeta.link) {

                }
            }
            return next(null, "rando");        
        })
    },
    function second(input, next){
        console.log('second: input:', input);
        return next('something bad happened');
    }
],
function(err){
    console.log('waterfall done: err:',err);
});