What does npm start do in nodejs

When you issue the com­mand npm start from the root direc­tory of your nodejs project, 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.
Npm scripts are pretty use­ful because they let you group together and run log­i­cally related com­mands. e.g. lin­ters, mini­fiers etc.