This script will find javascript modules that aren't currently used in your application.

find-dead-js-modules.sh

#!/bin/bash

# Make the script fail on the first error encountered.
set -euo pipefail

# Create a temp folder that we can use to store files in.
if [ "$(uname -s)" = "Darwin" ]; then
  tmp_dir=$(mktemp -d -t find-dead-modules.XXXXXXXX)
else
  tmp_dir=$(mktemp -d --tmpdir find-dead-modules.XXXXXXXX)
fi

# Make sure that the temp folder is removed on exit.
trap 'rm -rf "$tmp_dir"' EXIT INT QUIT TERM

touch "${tmp_dir}/all_modules.txt"
touch "${tmp_dir}/all_used_modules.txt"

# Find all javascript modules.
#
# We do this by listing all files, stripping out stuff that isn't part of the
# module "name" ("/index.js*", file suffix, path, etc). We also exclude a few
# utility modules that would otherwise be incorrectly flagged as dead.
echo "Finding all JavaScript files..."
find app/assets -name "**.js" -o -name "**.jsx" \
  | sed 's/\/index\.js$//' \
  | sed 's/\/index\.jsx$//' \
  | xargs -I % basename % .js \
  | xargs -I % basename % .jsx \
  | sort -u \
  >> "${tmp_dir}/all_modules.txt"

# Find all modules matching:
#
#   //= require ...
#   require('...')
#   import ... from '...'
#   import '...'
#   javascriptPaths['...']
#
# We exclude certain folders and files from the grep. The most important ones to
# ignore are specs, otherwise we would treat modules that are required only by
# their tests as "alive".
echo "Finding uses of JavaScript files in JavaScript..."
pcregrep -r -M --no-filename --only-matching \
  --buffer-size=1048576 \
  --exclude-dir=".git" \
  --exclude-dir="coverage" \
  --exclude-dir="node_modules" \
  --exclude-dir="public" \
  --exclude-dir="spec" \
  --exclude-dir="tmp" \
  --include="\.jsx?$" \
  "(//=\s*require\s+|require\(\s*'|import\s+'|from\s+'|javascriptPaths\[')\S+" . \
  | cut -d\' -f2 \
  | xargs -I % basename % .js \
  | xargs -I % basename % .jsx \
  >> "${tmp_dir}/all_used_modules.txt"

# Find all modules currently being used in Ruby via:
#
#   add_js_package
#   add_js_file
#   add_hypernova_package
#   render_react_component
#   render_react_component_or_mock_payload
#   javascript_path
#   javascript_include_tag
#   File.read
echo "Finding uses of JavaScript files in Ruby..."
pcregrep -r -M --no-filename --only-matching \
  --buffer-size=1048576 \
  --exclude-dir=".git" \
  --exclude-dir="coverage" \
  --exclude-dir="node_modules" \
  --exclude-dir="public" \
  --exclude-dir="spec" \
  --exclude-dir="tmp" \
  --include="\.e?rb$" \
  "(add_js_package|add_js_file|add_hypernova_package|render_react_component|render_react_component_or_mock_payload|javascript_path|javascript_include_tag|File\.read)\b\s*[:\(]?['\"]?\S+" . \
  | cut -d\' -f2 \
  | cut -d\" -f2 \
  | cut -d: -f2 \
  | xargs -I % basename % .js \
  | xargs -I % basename % .jsx \
  >> "${tmp_dir}/all_used_modules.txt"

# Sorting needs to happen to make the `comm` call below work the way we want it
# to.
echo "Sorting list..."
sort -u "${tmp_dir}/all_used_modules.txt" \
  --output="${tmp_dir}/all_used_modules.txt"

# Use `comm` to give us a list of all modules that aren't used.
echo "Finding unused files..."
UNUSED_MODULES=$(comm -23 \
  "${tmp_dir}/all_modules.txt" \
  "${tmp_dir}/all_used_modules.txt")

if [ -z "$UNUSED_MODULES" ]; then
  exit 0
else
  echo "These modules may not be used:"
  echo ""
  echo "$UNUSED_MODULES"
  exit 1
fi

Oh nice! I might be able to make it faster by combining greps and excluding more directories, but it is fast enough for my purpose right now. Also, too many false positives to set it up as a blocker to anything.