Transpiling ES6 and launching Express app upon file change - node.js

Question

I'm developing an Express app. The code uses ES6 import statements, so it needs to be transpiled by Babel in ES5 code so it can be run with Node. The ES6 source code is stored in my src directory, and the compiled code is placed in my dest directory.
I would like an npm command that I can run that will watch for any changes in my src directory, transpile the project into dest, and then restart the Express app.
Below is my solution so far (I run npm run server:watch). It uses chokidar-cli to execute the server:build and server:run scripts whenever any js file changes within my src directory. This works perfectly except that the Express app does not die when the code is transpiled again, so I get a 'port in use' exception when I try to run server:watch again.
// package.json
"scripts": {
"server:clean": "rm -f dist/*",
"server:build": "npm run server:clean && ./node_modules/.bin/babel ./src --experimental --source-maps-inline -d ./dist",
"server:run": "nodemon dist/app.js --watch",
"server:watch": "chokidar 'src/*.js' -c 'npm run server:build && npm run server:run'"
}
Am I on the right track here? What is the best solution to this sort of problem?
(For clarification, I am talking about transpiling server-side code, so webpack or any other module bundler would be inappropriate here)

Answer 1

nodemon is watching your dist files, and is never terminating. With your watch script, you're launching a second instance of nodemon every time there's a file change, hence the "port in use" error.
You need to launch nodemon once, and keep in running in the background while your watch script is running. This is most easily done with https://github.com/mysticatea/npm-run-all.
Install it by running:
npm install npm-run-all --save-dev
on the command line.
Then you'd edit your scripts to:
// package.json
"scripts": {
"server:clean": "rm -f dist/*",
"server:build": "npm run server:clean && ./node_modules/.bin/babel ./src --experimental --source-maps-inline -d ./dist",
"server:run": "nodemon dist/app.js --watch",
"server:watch": "chokidar 'src/*.js' -c 'npm run server:build'",
"server:dev": "run-p server:run server:watch"
}
Then, you simply run npm run server:dev on the command line to start up your server via nodemon, and start the watch/rebuild process, all in one command.
Side note: If you're developing on Windows, you won't be able to use single quotes in your scripts, you have to use double-quotes, escaped with \" in the JSON.