Sidestep

A simple control-flow library for node.JS that makes parallel execution, serial execution, and error handling painless.

Original credit goes to Tim Caswell for developing the brilliant Step library (https://github.com/creationix/step), which Sidestep was forked from.

How to install

Simply copy or link the lib/sidestep.js file into your $HOME/.node_libraries folder.

How to use

The step library exports a single function I call Sidestep. It accepts any number of functions as arguments and runs them in serial order using the passed in this context as the callback to the next step.

Sidestep(
  function readSelf() {
    fs.readFile(__filename, this);
  },
  function capitalize(err, text) {
    if (err) throw err;
    return text.toUpperCase();
  },
  function showIt(err, newText) {
    if (err) throw err;
    console.log(newText);
  }
)();

Notice that we pass in this as the callback to fs.readFile. When the file read completes, step will send the result as the arguments to the next function in the chain. Then in the capitalize function we're doing synchronous work so we can simple return the new value and Step will route it as if we called the callback.

The first parameter is reserved for errors since this is the node standard. Also any exceptions thrown are caught and passed as the first argument to the next function. As long as you don't nest callback functions inline your main functions this prevents there from ever being any uncaught exceptions. This is very important for long running node.JS servers since a single uncaught exception can bring the whole server down.

Also there is support for parallel actions:

Sidestep(
  // Loads two files in parallel
  function loadStuff() {
    fs.readFile(__filename, this.parallel());
    fs.readFile("/etc/passwd", this.parallel());
  },
  // Show the result when done
  function showStuff(err, code, users) {
    if (err) throw err;
    console.log(code);
    console.log(users);
  }
)();

Here we pass this.parallel() instead of this as the callback. It internally keeps track of the number of callbacks issued and preserves their order then giving the result to the next step after all have finished. If there is an error in any of the parallel actions, it will be passed as the first argument to the next step.

Also you can use group with a dynamic number of common tasks.

Sidestep(
  function readDir() {
    fs.readdir(__dirname, this);
  },
  function readFiles(err, results) {
    if (err) throw err;
    // Create a new group
    var group = this.group();
    results.forEach(function (filename) {
      if (/\.js$/.test(filename)) {
        fs.readFile(__dirname + "/" + filename, 'utf8', group());
      }
    });
  },
  function showAll(err , files) {
    if (err) throw err;
    console.dir(files);
  }
)();

Note that we both call this.group() and group(). The first reserves a slot in the parameters of the next step, then calling group() generates the individual callbacks and increments the internal counter.

Error handling

Sidestep supports passing in an error handler which will automatically be called (and skip the rest of the steps) if any of the steps calls the next step with an error parameter.

The error handler currently only works with serial steps, and is not supported by parallel steps.

Sidestep(
  function readSelf() {
    fs.readFile(__filename, this);
  },
  function capitalize(err, text) {
    return text.toUpperCase();
  },
  function showIt(err, newText) {
    console.log(newText);
  }
).error(function(err) {
	console.log("Error:", err);
})();

Now, it's no longer necessary to handle the errors in every Step, as they will all be handled by the trailing error function.