CODEWITHSUNDEEP
node.jspromisebluebird
Vineet
Vineet

Reputation: 1037

Creating functions to chain promises correctly

I am trying to get Promise chaining working for me correctly.

I believe the problem boils down to understanding the difference between:

promise.then(foo).then(bar);

and:

promise.then(foo.then(bar));

In this situation I am writing both foo and bar and am trying to get the signatures right. bar does take a return value that is produced by foo.

I have the latter working, but my question is what do I need to do to get the former working?

Related to the above is the full code (below). I don't have the different logs printed in the order I am expecting (expecting log1, log2, log3, log4, log5, but getting log3, log4, log5, log1, log2). I am hoping as I figure the above I will get this working right as well.

var Promise = require('bluebird');

function listPages(queryUrl) {
  var promise = Promise.resolve();

  promise = promise
    .then(parseFeed(queryUrl)
      .then(function (items) {

      items.forEach(function (item) {
        promise = promise.then(processItem(transform(item)))
                    .then(function() { console.log('log1');})
                    .then(function() { console.log('log2');});
      });

    }).then(function() {console.log('log3')})
  ).then(function() {console.log('log4')})
  .catch(function (error) {
    console.log('error: ', error, error.stack);
  });
  return promise.then(function() {console.log('log5');});
};

Upvotes: 2

Views: 51

Answers (1)

Bergi
Bergi

Reputation: 665486

What is the difference between promise.then(foo).then(bar); and promise.then(foo.then(bar));?

The second one is simply wrong. The then method takes a callback as its argument, not a promise. That callback might return a promise, so the first one is equivalent to

promise.then(function(x) { return foo(x).then(bar) })

(assuming that foo returns a promise as well).

Your whole code appears to be messed up a bit. It should probably read

function listPages(queryUrl) {
    return parseFeed(queryUrl)
    .then(function (items) {
        var promise = Promise.resolve();
        items.forEach(function (item) {
            promise = promise.then(function() {
                console.log('log1');
                return processItem(transform(item));
            }).then(function() {
                console.log('log2');
            });
        });
        return promise;
    }).then(function() {
        console.log('log3')
    }, function (error) {
        console.log('error: ', error, error.stack);
    });
}

Upvotes: 2

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