Show divergence from create-react-app as a diff.

diff-create-react-app.sh

#!/usr/bin/env bash
#
# Determine divergence from create-react-app!
# Use this if you've ejected from create-react-app and want to see how its
# latest output would differ in key areas (package.json, config, scripts).
#
# - Assumes you can run create-react-app, so make sure it's installed.
# - Only shows files you've modified or removed from create-react-app.
# - Runs $FORMAT_COMMAND below on the create-react-app directory so formatting
#   differences don't show up. Use something like Prettier, eslint --fix, etc.

set -e # Fail on error.
set -x # Show what we're doing.

APP_DIR="$PWD"
ESLINT="$APP_DIR/node_modules/.bin/eslint"
FORMAT_COMMAND=( "$ESLINT" --config "$APP_DIR/.eslintrc.js" --fix . )

# Make a new directory in which to run create-react-app.
NEW_APP_DIR="$(mktemp -d)/new-react-app"

# Note that we added /new-react-app onto the temp directory above.
# That's to ensure that create-react-app likes our directory.
mkdir -p "$NEW_APP_DIR"

# Run it!
create-react-app "$NEW_APP_DIR"

# Eject!
cd "$NEW_APP_DIR"
yes | head -n 1 | yarn run eject

# Remove files we don't care about diffing. This is easier than getting
# `git diff` to exclude them.
rm -rf .gitignore README* node_modules package-lock.json public src yarn.lock

# Run our local formatting config on the create-react-app output so that it's
# minimally different from ours.
"${FORMAT_COMMAND[@]}"

# Come back to our app so the remaining commands are based here.
cd "$APP_DIR"

# Make the new app's package.json look like ours, except for devDependencies
# and a few config fields, so that only those show up in the diff.
node -e "
const newPath = process.argv[1] + '/package.json';
const thisApp = require('./package.json');
const newApp = require(newPath);
thisApp.dependencies = newApp.dependencies;
thisApp.devDependencies = newApp.devDependencies;
thisApp.jest = newApp.jest;
thisApp.babel = newApp.babel;
thisApp.eslintConfig = newApp.eslintConfig;
require('fs').writeFileSync(newPath, JSON.stringify(thisApp, null, 2) + '\n');
" "$NEW_APP_DIR"

# Show the diff!
git diff --no-index --diff-filter=MD --color "$NEW_APP_DIR" . 2>/dev/null | less -CFRX

set +x
echo
echo "For comparison, the new create-react-app output can be found in:"
echo "  $NEW_APP_DIR"
echo