"scripts": {
"server": " npm run watch --prefix server",
"client":" npm run start --prefix client",
"watch":" npm run client & npm run server",
"test": "echo \"Error: no test specified\" && exit 1"
},
npm run watch only runs the first command (npm run client) before & operator.
install this package:https://www.npmjs.com/package/npm-run-all and then "start": "npm-run-all -p watch-css start-js", where watch-css and start-js are npm scripts in my case