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Concepts: Behaviors
Javelin Technical Documentation (concepts)

Javelin behaviors help you glue pieces of code together.


Javelin behaviors provide a place for you to put glue code. For instance, when a page loads, you often need to instantiate objects, or set up event listeners, or alert the user that they've won a hog, or create a dependency between two objects, or modify the DOM, etc.

Sometimes there's enough code involved here or a particular setup step happens often enough that it makes sense to write a class, but sometimes it's just a few lines of one-off glue. Behaviors give you a structured place to put this glue so that it's consistently organized and can benefit from Javelin infrastructure.

Behavior Basics

Behaviors are defined with JX.behavior():

JX.behavior('win-a-hog', function(config, statics) {
  alert("YOU WON A HOG NAMED " + config.hogName + "!");

They are called with JX.initBehaviors():

  "win-a-hog" : [{hogName : "Ethel"}]

Normally, you don't construct the JX.initBehaviors() call yourself, but instead use a server-side library which manages behavior initialization for you. For example, using the PHP library:

$config = array('hogName' => 'Ethel');
JavelinHelper::initBehaviors('win-a-hog', $config);

Regardless, this will alert the user that they've won a hog (named Ethel, which is a good name for a hog) when they load the page.

The callback you pass to JX.behavior() should have this signature:

function(config, statics) {
  // ...

The function will be invoked once for each configuration dictionary passed to JX.initBehaviors(), and the dictionary will be passed as the

config## parameter. For example, to alert the user that they've won two hogs:

  "win-a-hog" : [{hogName : "Ethel"}, {hogName: "Beatrice"}]

This will invoke the function twice, once for each config dictionary. Usually, you invoke a behavior multiple times if you have several similar controls on a page, like multiple JX.Tokenizers.

An initially empty object will be passed in the statics parameter, but changes to this object will persist across invocations of the behavior. For example:

JX.initBehaviors('win-a-hog', function(config, statics) {
  statics.hogsWon = (statics.hogsWon || 0) + 1;

  if (statics.hogsWon == 1) {
    alert("YOU WON A HOG! YOUR HOG IS NAMED " + config.hogName + "!");
  } else {
    alert("YOU WON ANOTHER HOG!!! THIS ONE IS NAMED " + config.hogName + "!");

One way to think about behaviors are that they take the anonymous function passed to JX.behavior() and put it in a private Javelin namespace, which you access with JX.initBehavior().

Another way to think about them is that you are defining methods which represent the entirety of the API exposed by the document. The recommended approach to glue code is that the server interact with Javascript on the client only by invoking behaviors, so the set of available behaviors represent the complete set of legal interactions available to the server.

History and Rationale

This section explains why behaviors exist and how they came about. You can understand and use them without knowing any of this, but it may be useful or interesting.

In early 2007, Facebook often solved the "glue code" problem through the use of global functions and DOM Level 0 event handlers, by manually building HTML tags in PHP:

echo '<a href="#" '.
     'onclick="win_a_hog('.escape_js_string($hog_name).'); return false;">'.
     'Click here to win!'.

(This example produces a link which the user can click to be alerted they have won a hog, which is slightly different from the automatic alert in the other examples in this document. Some subtle distinctions are ignored or glossed over here because they are not important to understanding behaviors.)

This has a wide array of technical and architectural problems:

  • Correctly escaping parameters is cumbersome and difficult.
  • It resists static analysis, and is difficult to even grep for. You can't easily package, minify, or determine dependencies for the piece of JS in the result string.
  • DOM Level 0 events have a host of issues in a complex application environment.
  • The JS global namespace becomes polluted with application glue functions.
  • The server and client are tightly and relatively arbitrarily coupled, since many of these handlers called multiple functions or had logic in the strings. There is no structure to the coupling, so many callers relied on the full power of arbitrary JS execution.
  • It's utterly hideous.

In 2007/2008, we introduced jsprintf() and a function called onloadRegister() to solve some of the obvious problems:

onloadRegister('win_a_hog(%s);', $hog_name);

This registers the snippet for invocation after DOMContentReady fires. This API makes escaping manageable, and was combined with recommendations to structure code like this in order to address some of the other problems:

$id = uniq_id();
echo '<a href="#" id="'.$id.'">Click here to win!</a>';
onloadRegister('new WinAHogController(%s, %s);', $id, $hog_name);

By 2010 (particularly with the introduction of XHP) the API had become more sophisticated, but this is basically how most of Facebook's glue code still works as of mid-2011. If you view the source of any page, you'll see a bunch of onloadRegister() calls in the markup which are generated like this.

This mitigates many of the problems but is still fairly awkward. Escaping is easier, but still possible to get wrong. Stuff is a bit easier to grep for, but not much. You can't get very far with static analysis unless you get very complex. Coupling between the languages has been reduced but not eliminated. And now you have a bunch of classes which only really have glue code in them.

Javelin behaviors provide a more structured solution to some of these problems:

  • All your Javascript code is in Javascript files, not embedded in strings in in some host language on the server side.
  • You can use static analysis and minification tools normally.
  • Provided you use a reasonable server-side library, you can't get escaping wrong.
  • Coupling is reduced because server only passes data to the client, never code.
  • The server declares client dependencies explicitly, not implicitly inside a string literal. Behaviors are also relatively easy to grep for.
  • Behaviors exist in a private, structured namespace instead of the global namespace.
  • Separation between the document's layout and behavior is a consequence of the structure of behaviors.
  • The entire interface the server may invoke against can be readily inferred.

Note that Javelin does provide JX.onload(), which behaves like

onloadRegister()##. However, its use is discouraged.

The two major downsides to the behavior design appear to be:

  • They have a higher setup cost than the ad-hoc methods, but Javelin philosophically places a very low value on this.
  • Because there's a further setup cost to migrate an existing behavior into a class, behaviors sometimes grow little by little until they are too big, have more than just glue code, and should have been refactored into a real class some time ago. This is a pretty high-level drawback and is manageable through awareness of the risk and code review.