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

# 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.
find app/assets -name "**.js" -o -name "**.jsx" \
  | sed 's/\/index.*\.js.*//' \
  | sed 's/.*\/\(.*\)/\1/' \
  | sed 's/\(.*\)\.js.*/\1/' \
  | sed '/GenericContainer/d' \
  | sed '/StyleGuideExample/d' \
  | sort -u \
  > "${tmp_dir}/all_modules.txt"

# Find all modules currently being `require`d.
#
# We exclude certain folders and files from the grep. The most important ones to
# ignore are index-test.js*, index-diffux.js, etc. Otherwise we would treat
# modules that are required only by their tests as "alive".
grep -r "require('.*')" . \
  --exclude-dir="node_modules" \
  --exclude-dir=".git" \
  --exclude="**/index-*.js*" \
  --include="**.js*" \
  --no-filename \
  --only-matching \
  | cut -d\' -f2 \
  | sed 's/.*\/\(.*\)/\1/' \
  > "${tmp_dir}/all_used_modules.txt"

# Add files listed in webpack config.
#
# Not all files are referenced through a `require` statement. We also have to
# grab a few modules referenced in webpack config.
grep -r "assets\/javascripts/.*\.js" . \
  --include="webpack.*" \
  --only-matching \
  --no-filename \
  | cut -d'/' -f3 \
  | cut -d'.' -f1 \
  >> "${tmp_dir}/all_used_modules.txt"

# Sorting needs to happen to make the `comm` call below work the way we want it
# to.
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.
UNUSED_MODULES=$(comm -23 \
  "${tmp_dir}/all_modules.txt" \
  "${tmp_dir}/all_used_modules.txt")

if [ -z "$UNUSED_MODULES" ]; then
  exit 0
else
  echo "These modules are not in use:"
  echo "$UNUSED_MODULES"
  exit 1
fi

There are some brigade specific stuff in here, but in general this is generic enough to be used on any codebase.