Load Libraries in Skewer with Bower

I recently added support to Skewer for loading libraries on the fly using Bower’s package infrastructure. Just make sure you’re up to date, then while skewering a page run M-x skewer-bower-load. It will prompt for a package and version, download the library, then inject it into the currently skewered page.

Because the Bower infrastructure is so simple, Bower is not actually needed in order to use this. Only Git is required, configured by skewer-bower-git-executable, which it tries to configure itself from Magit if it’s been loaded.


Skewer comes with a userscript that adds a small toggle button to the top-right corner every page I visit. Here’s a screenshot of the toggle on this page.

When that little red triangle is clicked, the page is connected to Emacs and the triangle turns green. Click it again and it disconnects, turning red. It remembers its state between page refreshes so that I’m not constantly having to toggle.

It’s mainly for development purposes, but it’s occasionally useful to Skewer an arbitrary page on the Internet so that I can poke at it from Emacs. One habit that I noticed comes up a lot is that I want to use jQuery as I fiddle with the page, but jQuery isn’t actually loaded for this page. What I’ll do is visit a jQuery script in Emacs and load this buffer (C-c C-k). As expected, this is tedious and easily automated.

Rather than add specific support for jQuery, I thought it would be more useful to hook into one of the existing JavaScript package managers. Not only would I get jQuery but I’d be able to load anything else provided by the package manager. This means if I learn about a cool new library, chances are I could just switch to my *javascript* scratch buffer, load the library with this new Skewer feature, and play with it. Very convenient.

How it Works

There are a number of package managers out there. I chose Bower because of its emphasis on client-side JavaScript and, more so, because its infrastructure is so simple that I wouldn’t actually need to use Bower itself to access it. In adding this feature to Skewer, I wrote half a Bower client from scratch very easily.

The only part of the Bower infrastructure hosted by Bower itself is a tiny registry that maps package names to Git repositories. This host also accepts new mappings, unauthenticated, for registering new packages. The entire database is served up as plain old JSON.

To find out what versions are available, clone this repository with Git and inspect the repository tags. Tags that follow the Semantic Versioning scheme are versions of the package available for use with Bower. Once a version is specified, look at bower.json in the tree-ish referenced by that tag to get the rest of the package metadata, such as dependencies, endpoint listing, and description.

This is all very clever. The Bower registry doesn’t have to host any code, so it remains simple and small. It could probably be rewritten from scratch in 15-30 minutes. Almost all the repositories are on GitHub, which most package developers are already comfortable with. Package maintainers don’t need to use any tools or interact with any new host systems. Except for adding some metadata they just keep doing what they’re doing. I think this last point is a big part of MELPA’s success.

Bower’s Fatal Weaknesses

Unfortunately Bower has two issues, one of which is widespread, that seriously impacts its usefulness.

Dependency Specification

Even though Bower specifies Semantic Versioning for package versions, which very precisely describes version syntax and semantics, the dependencies field in bower.json is underspecified. There’s no agreed upon method for specifying relative dependency versions.

Say your package depends on jQuery and it relies on the newer jQuery 1.6 behavior of attr(). You would mark down that you depend on jQuery 1.6.0. Say a user of your package is also using another package that depends on jQuery, it’s using the on() method, which requires jQuery 1.7 or newer. It specifies jQuery 1.7.0. This is a dependency conflict.

Of course your package works perfectly fine with 1.7.0. It works fine at 1.6.0 and later. In other package management systems, you would probably have marked that you depend on “>=1.6.0” rather than just 1.6.0. Unfortunately, Bower doesn’t specify this as a valid dependency version. Some package maintainers have gone ahead and specified relative versions anyway, but inconsistently. Some use the “>=” prefix like I did above, some prefix with “~” (“about this version”), which is pretty useless.

And this leads into the other flaw.

Most Bower Packages are Broken

While some parts of Bower are underspecified, most packages don’t follow the simple specifications that already exist! That is to say, most Bower packages are broken. This is incredibly unfortunate because it means at least half of the packages can’t be loaded by this new Skewer feature.

How are they broken? As of this writing, there are 2,195 packages list in Bower’s registry.

The good news is that most of the important libraries, like jQuery and Underscore, work properly. I’ve also registered two of my JavaScript libraries, ResurrectJS and rng-js, so these can be loaded on the fly in Skewer.

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Chris Wellons