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Minifying Javascript using ESBuild with Flask-Assets

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I've been using Flask and its Flask-Assets package (which is a wrapper around the webassets package) for quite some time now without issue, but recently, I've been scratching an itch for ES2016 and beyond. One of my favorite features of ES2016+ is that modules (and importing them) are part of the native browser implementation! No more AMD, CommonJS, RequireJS, and the like. Much to my chagrin, however, the Flask-Assets default Javascript minifier, jsmin, doesn't seem to have great support for bundling modules. It does fine with minifying them, but bundling is outside of its wheelhouse. Enter: ESBuild.

I won't get into the particulars of ESBuild except to say that it does a good job of not only minifying Javascript, but also tree-shaking and managing a script's dependencies. It seemed like a great option for replacing jsmin, so I set about doing so. First things first, I needed to figure out how minifiers are even hooked up as far as the webassets package is concerned. I quickly discovered that a custom class with two methods and a single member can be slapped together in short order to produce a new minifier. Here, I have done just that with esbuild:

from os import dirname
from subprocess import PIPE, Popen, STDOUT
from webassets.filter import Filter, register_filter

class ESBuildFilter(Filter):

    "Minify and bundle Javascript using esbuild."

    name = 'esbuild'

    def input(self, _in, out, **kwargs):
        cwd = dirname(kwargs['source_path'])
        p = Popen(['esbuild', '–bundle', '–minify', '–format=esm'],
                  cwd=cwd, stdin=PIPE, stdout=PIPE, stderr=STDOUT)
        outputs, errors = p.communicate('utf-8'))

    def output(self, _in, out, **kwargs):


It pulls the current working directory out of the source_path argument, which is the filename of the file being processed by the minifier. It then executes the esbuild command with select parameters, acting against stdin and sending its data to stdout for capture. To use it, simply swap out any filter reference to jsmin with esbuild, like so:

{% assets filters='esbuild', output='js/file.min.js', 'js/file.js' %}
<script src="{{ ASSET_URL }}" type="module"></script>
{% endassets %}

Note the type="module", as this is necessary for testing and developing the scripts while they are un-minified. Otherwise, the use of the import keyword in the scripts will throw an error. Happy minifying!