JavaScript is a complicated language. One of the difficult areas is scope. We can think of scope as the world a particular line of code can see. In JavaScript, if we neglect to use the var keyword for a variable, that variable is seen by the whole world from any line of code. Some large JavaScript applications intentionally use global variables to organize the system. Alternatively, a better pattern has emerged in which modules are used to access the other parts of the system. At first glance, this appears to be similar to an import in Java, a #include in C/C++, or a require in Ruby. But there is a very important difference: in most modular JavaScript approaches, the module is a scope that can hold state which is shared between all consumers of that module.

Almost a year ago, I started working on a huge JavaScript web application. At one point, it surpassed 600,000 lines of JavaScript not including third party libraries. The application used a framework that allowed for the creation of controllers and views. When a controller was defined, it was attached to the global scope by, in effect, omitting the var keyword. If this controller needed to be used by other JavaScript code, one could simply use the global variable to access it. The use of globals tightly coupled the system together in ways which made it hard to write new code using test-driven development. The tests would need to mock out all of the globals accessed by the code under test. After working with the system for a couple of months, it was apparent to us that this two year old application felt like it had four to five years of technical debt.

The good news was that we were not the only ones who felt this way. While the framework was firmly entrenched in the application, the team began to chip away at the globals and the whole approach that encouraged creation of these globals. The first step was adding asynchronous module definition to the frameworks dependency management function with define and require functions. We took this old school code:

thieve ( 'subject_controller.js' , function () { ... });

And converted it to:

define ([ 'subject_controller.js' ], function ( SubjectController ) { ... });

Here SubjectController is defined by subject_controller.js. This makes it trivial to define SubjectController as a mock. However one of the problems is that now the mock has replaced the SubjectController for the entirety of the scope. Having to put all the tests that depend on the mock in one file and the other tests in a different file seemed like a serious limitation, particularly when one might want to use different mocks for the same module.

In response, the team added a factoryFor function that would behave like so:

var subjectControllerFactory = thieve . factoryFor ( 'subject_controller.js' );

Now if subjectcontroller.js_ depends on modules like so:

define ([ 'jquery' , 'subject' , 'grinder' ], function ( $ , Subject , Grinder ) { ... });

We can, with the factory, inject our own definitions for these modules:

var controller = subjectControllerFactory ( mockJquery , mockSubject , grinder );

While these examples are simple, they are obfuscating some aspects. So lets rewind and see a practical example.

browser.js:

1 define ([ 'jquery' ], function ( $ , userAgent ) { 2 3 var ua = userAgent || navigator . userAgent ; 4 ua = ua . toLowerCase (); 5 6 var match = /(chrome)[ \/]([\w.]+)/ . exec ( ua ) || 7 /(webkit)[ \/]([\w.]+)/ . exec ( ua ) || 8 /(opera)(?:.*version|)[ \/]([\w.]+)/ . exec ( ua ) || 9 /(msie) ([\w.]+)/ . exec ( ua ) || 10 ua . indexOf ( "compatible" ) < 0 && /(mozilla)(?:.*? rv:([\w.]+)|)/ . exec ( ua ) || []; 11 12 var engine = match [ 1 ]; 13 var version = match [ 2 ]; 14 15 return { 16 engine : function () { 17 return engine ; 18 }, 19 20 version : function () { 21 return version ; 22 } 23 }; 24 });

browser_spec.js:

1 require ([ 'jquery.js' , 'browser.js' ], function ( jQuery ) { 2 describe ( "Browser" , function () { 3 4 //... 5 6 it ( 'can parse the IE10 user agent string' , function () { 7 var browserFactory = thieve . factoryFor ( 'browser.js' ); 8 var Browser = browserFactory ( jQuery , "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)" ); 9 10 expect ( Browser . engine ()). toBe ( 'msie' ); 11 expect ( Browser . version ()). toBe ( '10.0' ); 12 }); 13 14 //... 15 }); 16 });

The point of this code is to check the web browser engine and version. The code in browser.js is a slightly modified version of the deprecated jQuery.browser. Remember the part about how modules are basically shared scope? We can see this in browser.js, because any consumer of the module is going to get the exact same instance as the first consumer. The first time the code is consumed, lines 3-13 are executed. These extract the engine and the version. The scope within the module now has engine and version defined. All consumers after the first will have only the return value supplied to them. This return value is an anonymous object with engine and version methods. These methods are run within the scope of the module so they will of course have access to the already set variables.

Moving on to the test, it becomes clear that we need to be able to inject a userAgent string that typically would be accessed from the current browser via navigator.userAgent (as it is done on line 3 of browser.js). After getting the factory for browser , we inject jQuery and a known userAgent string for Internet Explorer 10.

At this point, I hope that it is clear what a JavaScript module is in the context of asynchronous module definition. The aspect of scope within the module shared across all consumers is an oddity, yet it has many benefits. If you want to try this approach, choose your implementation of asynchronous module definition carefully. Some offer access to the factory while others do not (in particular, require.js does not -- wire.js is a good place to start). The team that I am working with on the large web application has made great progress modularizing the application using this approach. An interesting benefit is that the modular approach encourages good practices like splitting up your code into smaller pieces that are not tightly coupled. As the work to remove globals has gained momentum, the mood of the team has turned from pessimism to optimism.