npm run start 沒定義也可以執行

When you issue the command npm start from the root directory of your Node JS project, the node will look for a scripts object in your package.json file. If found, it will look for a script with the key start and run the command specified as its value.

If your package.json does not have any scripts object or if the scripts object does not have a start key, it will run the command node server.js instead.

沒定義就會是

node server.js