javascript - Serial execution with Q promises - Stack Overflow

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I think I'm misunderstanding how Q promises work. I want my first promise to resolve before the next one starts, but that's not happening. Here is my code:

var Q = require('q');

function doWork(taskName) {
  var deferred = Q.defer();
  console.log('starting', taskName);
  setTimeout(function() { 
    console.log('done with', taskName);
    deferred.resolve(); 
  });

  return deferred.promise;
}

doWork('task one')
  .then(doWork('task two'))
  .then(function() { console.log('all done'); });

This code produces:

$ node test.js 
  starting task one
  starting task two
  done with task one
  done with task two
  all done

I would hope that it produces:

$ node test.js 
  starting task one
  done with task one
  starting task two
  done with task two
  all done

What am I doing wrong?

I think I'm misunderstanding how Q promises work. I want my first promise to resolve before the next one starts, but that's not happening. Here is my code:

var Q = require('q');

function doWork(taskName) {
  var deferred = Q.defer();
  console.log('starting', taskName);
  setTimeout(function() { 
    console.log('done with', taskName);
    deferred.resolve(); 
  });

  return deferred.promise;
}

doWork('task one')
  .then(doWork('task two'))
  .then(function() { console.log('all done'); });

This code produces:

$ node test.js 
  starting task one
  starting task two
  done with task one
  done with task two
  all done

I would hope that it produces:

$ node test.js 
  starting task one
  done with task one
  starting task two
  done with task two
  all done

What am I doing wrong?

• javascript
• q

Share Improve this question Follow edited Mar 11, 2013 at 23:43 Nick Heiner asked Mar 11, 2013 at 23:29 Nick HeinerNick Heiner 123k192192 gold badges486486 silver badges704704 bronze badges

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3 Answers 3

Sorted by: Reset to default 7

This works:

doWork('task one')
  .then(function() {
    return doWork('task two')
  })
  .then(function() {
    console.log('all done'); 
  });

That makes sense - just calling doWork directly in then() will fire off the timeout immediately, instead of giving Q a chance to wait until task one is plete.

The reason is that the doWork needs to be referenced as a function. If you want to reference a function inside the '.then' then you just give the function name, you don't pass the parameters. When the parser sees .then(doWork('taskTwo'))it will run doWork('taskTwo') BEFORE the .then is even evaluated. It's trying to bind the function parameter.

In this case, if you return the parameter for the next task in the resolved promise of the previous task then the parser will call doWork with the correct parameter and in the correct order.

var Q = require('q');
function doWork(taskNum) {
    var deferred = Q.defer();
    console.log('starting', taskNum);
    setTimeout(function() { 
      console.log('done with task', taskNum);
      deferred.resolve(++taskNum); 
    });

    return deferred.promise;
}

doWork(1)
.then(doWork)
.then(function(lastTaskNum) { console.log('all done'); });

Sample code using q and request

var Q       = require('q'),
    request = require('request'),
    api     = {};


api.post = function (options) {

    var deferred = Q.defer();

    request.post(options, function (error, response, body) {
            error ? deferred.reject(error) : deferred.resolve(body);
    });

    return deferred.promise;
};

api.get = function (options) {
    var deferred = Q.defer();

    request.post(options, function (error, response, body) {
            error ? deferred.reject(error) : deferred.resolve(response);
    });

    return deferred.promise;
}

api
    .post({url: 'https://foo.'})
    .then(function (body) {
            console.log(body);
            return api.get({url: 'http://myspace.hell'});
    }, function (error) {
            //error handling logic
    })
    .then(function (response) {
            console.log(response);
    }, function (error) {
            //error handling logic
    })
    .done(); //when you are done

In the code above, you can see that I define 2 API methods: get and post.

I'm using the request library.

• my post api method, resolves the promise with the body of response object returned by request.post()
• my get api method, resolves the promise with the response of the request.get() call

You can see exactly how you can chain these 2 api call using the promises.

In the first then I return the second promise, so that I can chain the promise.

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