Document custom COMPRESS_FILTER implementation

[dalescher]

I am trying to use terser https://github.com/terser/terser with django-compressor. It's working quite fine except that I have an issue I do not understand.

Once compressed, some of my output files are missing some data compared to the originals (some functions disappear especially). But when I run terser from the command line (outside of django) with the same argument, everything is fine. Here is what I use from the command line to get the right results:

terser <input_file> -c -m --keep-fnames --toplevel -o <output_file>

and here the equivalent arguments in settings.py :

COMPRESS_TERSER_JS_ARGUMENTS = "-c -m --keep-fnames --toplevel"

Here are the settings I have:

settings.py

COMPRESS_ENABLED = True
COMPRESS_OFFLINE = True
COMPRESS_CSS_HASHING_METHOD = None
COMPRESS_FILTERS = {
    'js': [
        'appName.compressor.filters.terser.TerserJSFilter',
    ]
}
COMPRESS_TERSER_BINARY = "terser"
COMPRESS_TERSER_JS_ARGUMENTS = "-c -m --keep-fnames --toplevel"
COMPRESS_URL = STATIC_URL
COMPRESS_OUTPUT_DIR = 'CACHE'

and here is the custom TerserJSFilter I have created (inspired from ClosureCompilerFilter):

terser.py in appName/compressor/filters folder

from compressor.conf import settings
from compressor.filters import CompilerFilter

class TerserJSFilter(CompilerFilter):
    command = "{binary} {args}"
    options = (
        ("binary", settings.COMPRESS_TERSER_BINARY),
        ("args", settings.COMPRESS_TERSER_JS_ARGUMENTS),
    )

Am I implementing terser correctly or missing something?
Is there a way to get more detailed logs from django-compressor in order to have a better understanding of what's going on?
Does someone have an idea about why I have some data not taken into account/deleted in my output compressed files?

Thank you in advance

[dalescher]

ok, my bad. Looks like my issue comes from terser, not from django-compressor. Looks like using --toplevel argument has some nice effects I do not understand yet.

I prefer to not delete my post since I have not found any information about how to implement a custom filter like terser with django-compressor. So it might useful to others.

If you have any hint about terser behavior with --toplevel and why some functions and parts of code are deleted please let me know.

EDIT: Finally found why some code was remove. "-c" argument, which stands for "--compress" is also performing dead code removal. Since I am using some helper functions for other scripts, they were considered as dead code because not used in the scope of the analyzed file. nothing to do with --toplevel directly. To solve this, I passed the following option to compress: 'dead_code=false, unused=false'. My final arguments setting for TERSER in settings.py is now:

COMPRESS_TERSER_JS_ARGUMENTS = "-c dead_code=false,unused=false -m --keep-fnames --toplevel --ecma 2020"