How can I exit a promise sequence when the array has been processed?

Question

I've implemented the following code example in a script which loops over an array of results and makes an http request for each one.

function sequence(array, callback) {
    return array.reduce(function chain(promise, item) {
        return promise.then(function () {
            return callback(item);
        });
    }, Promise.resolve());
};

var products = ['sku-1', 'sku-2', 'sku-3'];

sequence(products, function (sku) {
    return getInfo(sku).then(function (info) {
        console.log(info)
    });
}).catch(function (reason) {
    console.log(reason);
});

function getInfo(sku) {
    console.log('Requested info for ' + sku);
    //assume this function makes an http request and returns a  promise
}

The script does not exit once all items in the array have been processed. How would I adapt it so that this happens?

My implementation makes an API call and returns a promise:

....

}).then(function(results){
sequence(results, function(result){

    checkSpam(result).then(function(akismet){
        if(akismet) {
            //console.log('spam returned from checkspam');
            write_spam_to_file(result);
        } else {

            //console.log('no spam returned from checkspam');
        }
    })
});

}).catch(function (e) {
    console.log('Error occurred:');
    console.log(e);
});


function sequence(array, callback) {
    return array.reduce(function chain(promise, item) {
        return promise.then(function () {
            return callback(item);
        });
    }, Promise.resolve());
}

function checkSpam(entry) {

    var akismetPromise = new Promise(function resolver(resolve, reject) {
        akismet.checkSpam({
            user_agent: 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:40.0) Gecko/20100101 Firefox/40.1',
            referrer: 'http://google.com',
            user_ip: entry.ip,
            comment_author: entry.poster,
            comment_content: entry.message,
            comment_type: 'forum-post'


        }, function (err, spam) {
            if (err) reject(err);
            resolve(spam);
        });

    });
    return akismetPromise;

}

What exactly do you mean by "does not exit"? You mean node doesn't stop running? That's hardly caused by the promises, rather by `akismet` or whatever you are using. — Bergi, Mar 23 '16 at 19:26
@Bergi I am new to promises so wasn't sure if I had done something incorrectly. I've added a bit more code in case that sheds any light. Thanks for taking a look. — codecowboy, Mar 23 '16 at 19:42
When looping through arrays you can return false from within the loop method to stop the iteration. — Silkster, Mar 23 '16 at 19:52

score 3 · Accepted Answer · edited May 23 '17 at 12:23

3

In contrast to your first snippet, your actual code lacks a bunch of return keywords:

    …
}).then(function(results) {
    return sequence(results, function(result) {
//  ^^^^^^
        return checkSpam(result).then(function(akismet) {
//      ^^^^^^
            if (akismet) {
                return write_spam_to_file(result);
//              ^^^^^^ // assuming the write is asynchronous and returns a promise?
            }
        });
    });
}) …

If you don't always return a promise from asynchronous functions (including then callbacks), the promise chain doesn't know what to wait for, and executes the next callback without waiting for any results.

edited May 23 '17 at 12:23

Community

1
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answered Mar 23 '16 at 20:15

Bergi

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we need a canonical on forgotten returns these questions are returning a ton of times. "Promise not waiting for action" or something like that, the problem is that is sounds a little too generic... – Benjamin Gruenbaum Mar 23 '16 at 21:33
@BenjaminGruenbaum: Yeah, but the next canonical I'm going to write is about asynchronous loops :-) (unless we already have a good one?) – Bergi Mar 23 '16 at 21:53

How can I exit a promise sequence when the array has been processed?

1 Answers1