Node.js concat array after async.concat()

Question

I have an array, I need to recompile with the use of some edits. I do it with the help of async.concat(), but something is not working. Tell me, where is the mistake?

async.concat(dialogs, function(dialog, callback) {
    if (dialog['viewer']['user_profile_image'] != null) {
        fs.exists(IM.pathToUserImage + dialog['viewer']['user_profile_image'].replace('%s', ''), function(exits) {
            if (exits) {
                dialog['viewer']['user_profile_image'] = dialog['viewer']['user_profile_image'].replace('%s', '');
            }
            callback(dialog);
        });
    }
}, function() {
    console.log(arguments);
});

In my opinion, everything is logical. callback is invoked immediately after the first iteration. But how can I send the data after the completion of processing the entire array?

Thank you!

What does the console.log call log? What's unexpected with the result? — Bergi
– Bergi, Commented Jul 17, 2013 at 14:18
Ah, that's because you're "throwing" an error. Use the second argument to callback instead. — Bergi
– Bergi, Commented Jul 17, 2013 at 14:33

go-oleg · Accepted Answer · 2013-07-17 14:19:27Z

3

Instead of callback(dialog);, you want

callback(null,dialog);

because the first parameter to the callback function is an error object. The reason that console.log(arguments) is getting called after the first iteration is because async thinks an error occurred.

answered Jul 17, 2013 at 14:19

go-oleg

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4 Comments

RomanGor Over a year ago

I think it is not fundamentally important. But even with that there is a problem, if I pass the callback in the first argument null, nothing inside callback it does not work, even console.log(arguments);

go-oleg Over a year ago

@RomanGorbatko: The documentation also says that the second value that is passed to the callback should be an array. It looks like dialog is an object?

RomanGor Over a year ago

No, dialog is a object. But, if done well: callback (null, [dialog]), we get the same thing - nothing

RomanGor Over a year ago

If I as the first argument is null then the inside callback, nothing happens, but if you pass something like 123 - works.

Leonid Beschastny · Accepted Answer · 2013-07-17 16:47:59Z

2

I solved the problem but did not understand its meaning. The problem was due to the element which was null instead of the processed value. The program broke off at this point, but do not throw away any errors / warnings.

async.map(dialogs, function(dialog, callback) {
    if (dialog['viewer']['user_profile_image'] == null) {
        dialog['viewer']['user_profile_image'] = IM.pathToUserImage;
    }
    fs.exists(IM.pathToUserImage + dialog['viewer']['user_profile_image'].replace('%s', ''), function(exits) {
        if (exits) {
            dialog['viewer']['user_profile_image'] = dialog['viewer']['user_profile_image'].replace('%s', '');
        }
        callback(null, dialog);
    });
}, function(err, rows) {
    if (err) throw err;
    console.log(rows);
});

edited Jul 17, 2013 at 16:47

Leonid Beschastny

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answered Jul 17, 2013 at 15:17

RomanGor

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7 Comments

robertklep Over a year ago

You always have to call the callback function passed by async to your handler function. In your initial example, you didn't call it if dialog['viewer']['user_profile_image'] happened to be null. That will probably have resulted in undefined behavior.

Leonid Beschastny Over a year ago

Why are you using async.concat() here and not async.map()?

RomanGor Over a year ago

@LeonidBeschastny Because I need to collect all of the elements of the array into one. But, frankly, I do not quite understand the difference between .concat and .map.

RomanGor Over a year ago

@robertklep I was confused what node.js did not throw any error.

Leonid Beschastny Over a year ago

async.map maps each element of an array into a new one, resulting in a new array of the same length. async.concat maps each element of an array into an array of sub-elements and then concatenates all them into one array. So, with async.concat you can produce an array of any length, while async.map will always produce an array of the same length as the original one.

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Anurag Sindhu · Accepted Answer · 2020-05-20 03:54:52Z

0

Though I am a little late to post this answer, I see none of us has used .concat function in the way it supposed to be used.

I have created a snippet that says the proper implementation of this function.

let async = require('async');
async.concat([1, 2, 3], hello, (err, result) => {
    if (err) throw err;
    console.log(result); // [1, 3]
});

function hello(time, callback) {
    setTimeout(function () {
        callback(time, null)
    }, time * 500);
}

answered May 20, 2020 at 3:54

Anurag Sindhu

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