Development Bundling

When you're working on small experiments or demos, a convenient feature of jspm is that you don't need to run any form of setup task. jspm loads and transpiles everything in the browser, removing the reliance on OS-installed tooling.

The obvious problem you might notice is that it takes about 3 seconds to render the sample component here in Chrome with devtools open and 2 seconds in Firefox or Chrome with devtools disabled. While in theory it should be possible for this refresh to be well under a second, the reality is that web browsers are not currently optimized to handle high numbers of interleaved fetch / execution cycles.

This is due to the large number of requests made by React, which we now optimize for further development by switching to a development bundle workflow with watching to generate a single unminified bundle file to be loaded in the browser.

To add this improvement, we will use the jspm bundle command with the watch flag:

jspm bundle test.js -wid

where the flags here are:

  • --watch: Watch the module tree for file changes to then do a quick cached rebuild.
  • --inject: Ensure that the bundle is automatically loaded without any further changes necessary.
  • --development: Create a development bundle, without any production flags or optimizations, which are otherwise the default when creating a bundle.

This will then display:

     Building the bundle tree for test.js...


ok   build.js added to config bundles.
ok   Built into build.js with source maps, unminified.
Watchman:  Watchman was not found in PATH.  See for installation instructions
     Watching . for changes with Node native watcher...

Now any changes to any of the modules will automatically rebuild the bundle, refreshing the browser will then load the changes from that single bundle file for a quick page refresh.

For performance, it is advisable to close the Chrome devtools when reloading the page, before opening them again as there is a longer page load with devtools open.

For the fastest workflow it is worth installing Watchman as well. This lets jspm watch for file changes more efficiently.