Some observations on porting the QuickUI runtime from plain JavaScript to CoffeeScript

This post shares some highlights of the experience porting a non-trivial library from plain JavaScript to CoffeeScript in case other parties are considering a similar transition.

Yesterday’s announcement of QuickUI 0.9 mentioned that the framework source code has now been ported to CoffeeScript. The QuickUI framework is intended for plain JavaScript development as well; nothing in the change of source language changes that. But experimentation with the CoffeeScript language suggested there were enough advantages to the language that, going forward, it would be worth porting the runtime from plain JavaScript to CoffeeScript.

Overall, the port from plain to JavaScript to CoffeeScript went rather smoothly, and the bulk of it took about two days. The QuickUI runtime, quickui.js, is a reasonably complex JavaScript library, which is to say that it’s not a toy or trivial sample application. The last plain JavaScript version of the QuickUI runtime, quickui-0.8.9.js, was about 7700 lines of plain JavaScript (including comments), or about 60K, developed over the course of four and a half years.

Automatic translation with js2Coffee

The handy js2coffee conversion tool was used to kickstart the port. Kudos to Rico Sta. Cruz for this great tool.

  • The automatically translated CoffeeScript immediately passed 97% of the QuickUI unit test suite. The remaining 4 broken tests were do to a single issue related to translation of the “instanceof” keyword, which was easy enough to work around.
  • The one thing js2coffee doesn’t translate (yet) are comments, so these had to be copied over by hand. Tedious, but straightforward.
  • Similarly, the js2coffee output sometimes produced long lines that needed to be hand-broken for legibility. Again, a bit tedious but straightforward.
  • Once all unit tests passed, the unit tests themselves were ported to CoffeeScript by the same process.

After about a morning of work, a CoffeeScript-based quickui.js was functional. It passed all unit tests, and could actually be used to drive a non-trivial QuickUI-based body of code like the QuickUI Catalog.

Towards idiomatic CoffeeScript

After the mechanical port with js2coffee, various CoffeeScript idioms were applied incrementally to replace the standard JavaScript idioms with their more concise CoffeeScript versions. This took another day and half or so.

  • There was occasion to use pretty much all of CoffeeScript’s syntactic sugar. References to Foo.prototype.bar() were replaced with the more concise Foo::bar(). Closure variables to hold “this” for use in an event handler were replaced with CoffeeScript’s “=>” syntax. Etc., etc.
  • Because CoffeeScript can wrap a body of code in a single function closure, this no longer needed to be done by hand. A wrapping closure like that can complicate the management of a pile of plain JavaScript files. The closure will typically have to be created through a build process that includes a JavaScript fragment (to start the closure) before the real JavaScript files, and another fragment (to end the closure) afterwards. (The jQuery Makefile does this, for example.) CoffeeScript’s built-in support for a closure that spans multiple files finally made it easy enough to break up the quickui.js runtime from a single monolithic JavaScript file into a much saner and more manageable collection of CoffeeScript files. That is, while the same degree of manageability could have been achieved in plain JavaScript, CoffeeScript made it simple enough that it actually got done.
  • The QuickUI runtime itself doesn’t create many classes, but in some cases (e.g., the unit test suite), classes could be created via CoffeeScript’s concise class syntax. This took advantages of QuickUI’s new support for creating web user interface controls using CoffeeScript class syntax.
  • JavaScript “for” loops were replaced with CoffeeScript list comprehensions.

Idiomatic CoffeeScript iteration over jQuery objects

Speaking of “for” loops, it turns out that a good deal of the QuickUI runtime deals with looping over jQuery objects. QuickUI controls are a subclass of jQuery object, and when looping over them in plain JavaScript, it’s often convenient to use jQuery’s $.each() function. For example, this function invokes foo(), a jQuery method or plugin, on each element in a jQuery object:
var $elements = $(".someClass");
$elements.each( function( index, element ) {
    $( element ).foo();
});
Note that $.each() gives the callback the plain DOM element, so you have to wrap that element with $(element) to get a jQuery object you can then manipulate. To simplify that, QuickUI’s runtime has long had a helper function called eachControl() that gives the callback the element as a wrapped jQuery object. (In QuickUI’s case, it also ensures the control’s particular subclass of jQuery is used, so that you can directly manipulate the control with that control’s own specific API.) E.g.:
var $buttons = $(".BasicButton");
$buttons.eachControl( function( index, $button ) {
    $button.foo();
});
To take best advantage of CoffeeScript’s supports for looping constructs, a simple jQuery plugin was created to create an array that can directly be used by CoffeeScript’s “for” loop and list comprehensions. This plugin, called Control.segments(), converts a jQuery object that holds a number of elements into an array of jQuery objects that each hold a single (wrapped) element. The definition of segments() in CoffeeScript is trivial:
Control::segments = ->
  ( @constructor element for element in @ )
QuickUI defines segments() on the Control class so as not to pollute the general jQuery plugin namespace, but the above definition could just as easily be done as jQuery::segments to create a plugin that worked with any jQuery object. In any event, the result of applying segments() to a jQuery object is an array that can be directly iterated over, while at the same time preserving type information.
$button.foo() for $button in Control(".BasicButton").segments()
Here, the looping variable $button ends up holding an instanceof BasicButton (which is also an instanceof jQuery), so $button.foo() invokes BasicButton.prototype.foo().
This “for” loop feels more direct and idiomatic in CoffeeScript than the standard $.each() approach. (In fact, it’d be nice if $.each() were extended so that, if invoked without arguments, it returned an array just like segments() does here.) This segments() call can also be used in CoffeeScript list comprehensions, thereby replacing many situations in which $.map() is currently used. A jsperf experiment suggests the segments() approach performs roughly as well as the standard $.each(). The generated JavaScript for segments() does build a temporary array of results, but it avoids the need for the callback function and the accompanying closure.

Impressions

The new, CoffeeScript-based QuickUI source code gets compiled to a plain JavaScript file that’s essentially the same size as the handwritten JavaScript (61K vs 60K). The new runtime appears to perform and function just as well as the earlier plain JavaScript one, so QuickUI developers shouldn’t notice any difference. At the same time, the new CoffeeScript source code feels a lot tighter and easier to read and maintain.
This ability to write tighter code has already invited the successful implementation of a number of long-planned improvements to the runtime. It’s hard to say how many of those improvements were easier to tackle because of advantages in the CoffeeScript language itself, and how many were tackled just because CoffeeScript is a shiny, new tool. But as a general rule, it seems that CoffeeScript permits a programmer to more directly express their intention than one can do in JavaScript — and any language that can do that is a step forward.
Best of all, using any language like CoffeeScript that compiles to plain JavaScript enables a developer to finally break a hard dependence between language choice and the user’s browser. Now that QuickUI itself is written in CoffeeScript, it can take immediate advantage of improvements in CoffeeScript the day they appear, instead of waiting years for incremental JavaScript improvements to make their way through committee, into browsers, and into users’ hands.
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