Node.js Asynchronous Library Comparison - Q vs Async

Both libraries are good. I have discovered that they serve separate purposes and can be used in tandem.

Q provides the developer with promise objects, which are future representations of values. Useful for time travelling.

Async provides the developer with asynchronous versions of control structures and aggregate operations.

An example from one attempt at a linter implementation demonstrates a potential unity among libraries:

function lint(files, callback) {    // Function which returns a promise.    var getMerged = merger('.jslintrc'),        // Result objects to invoke callback with.        results = [];    async.each(files, function (file, callback) {        fs.exists(file, function (exists) {            // Future representation of the file's contents.            var contentsPromise,                // Future representation of JSLINT options from .jslintrc files.                optionPromise;            if (!exists) {                callback();                return;            }            contentsPromise = q.nfcall(fs.readFile, file, 'utf8');            optionPromise = getMerged(path.dirname(file));            // Parallelize IO operations.            q.all([contentsPromise, optionPromise])                .spread(function (contents, option) {                    var success = JSLINT(contents, option),                        errors,                        fileResults;                    if (!success) {                        errors = JSLINT.data().errors;                        fileResults = errors.reduce(function (soFar, error) {                            if (error === null) {                                return soFar;                            }                            return soFar.concat({                                file: file,                                error: error                            });                        }, []);                        results = results.concat(fileResults);                    }                    process.nextTick(callback);                })                .catch(function (error) {                    process.nextTick(function () {                        callback(error);                    });                })                .done();        });    }, function (error) {        results = results.sort(function (a, b) {            return a.file.charCodeAt(0) - b.file.charCodeAt(0);        });        callback(error, results);    });}

I want to do something potentially-blocking for each file. So async.each is the obvious choice. I can parallelize related operations per-iteration with q.all and reuse my option values if they apply to 2 or more files.

Here, Async and Q each influence the control flow of the program, and Q represents values resolving to file contents sometime in the future. The libraries work well together. One does not need to "choose one over the other".

Callback pyramids in your code can be simplified using promise composition and javascript lexical scoping.

removeItem: function (itemId) {  var removeRegexp = new RegExp('\\/stock\\.php\\?remove=' + itemId);  var found = false  var promise = getPage('/sock.php')  _.times(5, (i) => {    promise = promise.then((webpage) => {      if (found) return true      var removeMatch = webpage.match(removeRegexp)      var found = removeMath !== null      var nextPage = found ? removeMatch[0] : '/stock.php?p='+i+1      return Q.delay(1000).then(() => this.getPage(nextPage))    })  })  return promise.fail(console.log.bind(console))},

IMHO async should not be used in new javascript code. Promises are more composable, and allow for a lot more intutive code.

The primary reason why node did not use promises was because of performance concerns which have largely been addressed very well by libraries like Bluebird and Q.

As async/await syntax becomes more mainstream, promises will pave the way for code that looks very similar with synchronous code.

While this is still not an actual answer to my question (Q vs async), regarding my problem, I've found Selenium / WebDriverJs to be a viable solution.

driver.get('http://www.google.com');driver.findElement(webdriver.By.name('q')).sendKeys('webdriver');driver.findElement(webdriver.By.name('btnG')).click();driver.wait(function() {  return driver.getTitle().then(function(title) {    return title === 'webdriver - Google Search';  });}, 1000);

WebDriver uses a queue to execute promises sequentially, which helps immensely with controlling indentation. Its promises are also compatible with Q's.

Creating a sequence of promises is no longer an issue. A simple for-loop will do.

As for stopping early in a sequence, don't do this. Instead of using a sequence, use an asynchronous-while design and branch.