Question

127

Correct way to write loops for promise.

rated 0 times [ 133] [ 6] / answers: 1 / hits: 141826 / 10 Years ago, wed, july 9, 2014, 12:00:00

How to correctly construct a loop to make sure the following promise call and the chained logger.log(res) runs synchronously through iteration? (bluebird)

db.getUser(email).then(function(res) { logger.log(res); }); // this is a promise

I tried the following way (method from http://blog.victorquinn.com/javascript-promise-while-loop )

var Promise = require('bluebird');



var promiseWhile = function(condition, action) {

    var resolver = Promise.defer();



    var loop = function() {

        if (!condition()) return resolver.resolve();

        return Promise.cast(action())

            .then(loop)

            .catch(resolver.reject);

    };



    process.nextTick(loop);



    return resolver.promise;

});



var count = 0;

promiseWhile(function() {

    return count < 10;

}, function() {

    return new Promise(function(resolve, reject) {

        db.getUser(email)

          .then(function(res) { 

              logger.log(res); 

              count++;

              resolve();

          });

    }); 

}).then(function() {

    console.log('all done');

});

Although it seems to work, but I don't think it guarantees the order of calling logger.log(res);

Any suggestions?

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jenna

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3 Years ago

answered 10 Years ago longd · Accepted Answer

I don't think it guarantees the order of calling logger.log(res);

Actually, it does. That statement is executed before the resolve call.

Any suggestions?

Lots. The most important is your use of the create-promise-manually antipattern - just do only

promiseWhile(…, function() {

    return db.getUser(email)

             .then(function(res) { 

                 logger.log(res); 

                 count++;

             });

})…

Second, that while function could be simplified a lot:

var promiseWhile = Promise.method(function(condition, action) {

    if (!condition()) return;

    return action().then(promiseWhile.bind(null, condition, action));

});

Third, I would not use a while loop (with a closure variable) but a for loop:

var promiseFor = Promise.method(function(condition, action, value) {

    if (!condition(value)) return value;

    return action(value).then(promiseFor.bind(null, condition, action));

});



promiseFor(function(count) {

    return count < 10;

}, function(count) {

    return db.getUser(email)

             .then(function(res) { 

                 logger.log(res); 

                 return ++count;

             });

}, 0).then(console.log.bind(console, 'all done'));