Javascript Asynchronous Operations

Javascript is single threaded.  That's good.  But to take advantage of our ever increasing multi-processor world, we need to adopt functional solutions.  Thats where asynchronous operations come in.

Javascript applications that run in the browser or in node have asynchronous events occurring all the time.  This is especially true in node where most actions return results through asynchronous callbacks rather than direct return values.  This takes some time to get used to.

Consider the following recursive function/snippet:

var list = getList(); 
var nextListItem = function() {
    var item = list.pop();
 
    if (item) {
        // process the item...
        nextListItem();
    }
};
nextListItem();

The recursive list item processor will work fine if the list is a reasonable size. But, if the list is huge, then one has to worry about the size of call stack. With every iteration it grows by one call, so it may exceed memory limits and fail (in a very bad way) if there are too many iterations.

Now consider this small change:

var list = getList();
var nextListItem = function() {
    var item = list.pop();
 
    if (item) {
        setTimeout(function() {
            // process the item...
            nextListItem();
        }, 0);
    }
};
nextListItem();

The stack over-flow problem is now eliminated. Why? Because javascript's event system handles the recursion–not the call stack. When nextListItem runs, if 'item' is true (not null) then the timeout function is defined and pushed to the event queue, then the function exits–and the call stack is now clear. When the event queue runs it's timed' out event, the item is processed and nextListItem is recursively called.

Again, the method is processed from start to finish without a direct recursive call, so the call-stack is again clear–regardless of the number of iterations.

Now consider this example:

var userList,
    cleverProgrammerBonus = 10000;

var userUpdateCallback = function(err) {

    if (err) return updateCompleteCallback( err );
    nextUser();
};

var nextUser = function() {
    var user = userList.pop();
    if (user) {
        user.pay += cleverProgrammerBonus;
        dao.updateUser( user, userUpdateCallback );
    } else {

        updateCompleteCallback();

    }
};

var userFetchCallback = function(err, list) {
    if (err) return updateCompleteCallback( err );
    userList = list;
    nextUser();
};

dao.fetchUsers( userFetchCallback );

As with most functional, asynchronous solutions, it's best to start at the bottom–the dao fetch of users. The fetch callback sets the user list then invokes the recursive nextUser() to get the iteration started. Then the user is updated asynchronously, and when it completes updateUpdateCallback is invoked. This callback continues the recursion by invoking nextUser. This is repeated until the list is empty.

There isn't a need for setInterval ( or nextTick() ) because the dao calls to update the user are asynchronous–always invoked from the event queue, not the call stack. So the call stack remains static no matter how many users are in the list.

Its also important to realize that the function dao.updateUser doesn't really start working until nextUser has exited. It's queued to do some work, but not at the direction of the call stack–just the event queue.

Asynchronous event driven operations.  Another reason to love coding in javascript.

Javascript Coding Standards

I love javascript. I love how it's flexible enough to be as expressive as you want.  The power of closures used in the traditional way or simply as function pointers like in the old C days.

Javascript's flexible nature enables an infinite combination of styles.  From prototypical, IIFE, to classical where functions are actually called "classes" when no actual classes exist.

This flexibility can lead to endless discussions of "what's best" or "what's most efficient" and worse can lead to project code that can resemble a big steamy bowl of pasta with brownish marinara.  As a team leader, the last thing you want is to have to code walk dozens of coding styles.  As a coder and team member the last thing you what to do is traverse hundreds of lines of code that has bad indentation or horrible style.

So, as a manager, you need to adopt a strict coding standard and ruthlessly enforce it.

Coding Standard Objectives

Here is a short bullet list.  The standards should...

• match the team's current experience and talent,
• should closely follow at least one major published standard,
• should have justifications for each decision,
• and include team consensus.

Team Experience

Javascript isn't new, but it's new to many programmers.  If your team has years of java and c++ experience, the wide-open flexibility of javascript will come as a frustrating surprise.  If your team has a ruby influence, then you have a leg up.  Understanding closures, dynamic typing, etc will ease the transition to production ready javascript.  If your team is primarily PHP based, then they know a bit about javascript but probably don't understand its full capability.

For the Java Team

So lets begin with the java programming team--what standards would suite them best?  The choice is easy--use a classical implementation with factory generated components that implement IoC through parameter injection.  Think of it this way, every file is a class defined by a named function variable (the function is still technically anonymous).  Its injection is always through construction (think ruby opts), and always factory built.  Files are organized as pseudo-package folders that may (or may not) have package namespaces.

The coding style is very similar to the java specs.  K&R formatting, title cased classes, mixed case variables, no underscores.  Complete separation of concerns, i.e. MVC plus services, delegates and factories.  And, unit tests--either TDD or BDD for everything.  And not just the normal tests, but also tests to find bugs that the compiler would normally find.  (mocha is probably the best test platform for both client and server javascript today).

Published Standards

Douglas Crockford.  JSLint.  Javascript, the good parts.  If you can only read one javascript book, please read "the good parts".   Coding conclusions aren't drawn on what is best--just what the alternatives are.  But, he does describe what he feels are some best practices for coding style, indentation, curly braces, etc.  And, if you have the time, watch his video series on javascript.

For good published standards, start here:

• google standards.
• yahoo standards.

Standards you probably want stay away from:

• if you are writing applications, the don't follow these standards; utility modules maybe, but...
• M$ standards--known only to a few Seattle east-siders.

Our Standards Document

Here is a link to our current javascript coding standards.  For the most part, this is for client applications, but most parts apply to node/server projects as well.

Conclusions

My best advise is to find a good published standard, modify it for your team's abilities, then stick with it.  You will need to add rules as time goes on--because coders will always find differing ways to interpret the rules.  So be prepared for additions--not so much changes, just clarifications.

Happy coding...