So, let’s say you need to add new flag --watch to your script from package.json In this case to “test” script

"scripts": {
  "test": "jest"

Yes, you can add "test": "jest --watch" to your package.json, but that might be not very convinient, insted you can add this directly in terminal. But mind, that

$ npm test --watch

won’t work as expected because --watch will be considered as a part of npm command, not jest comman as we would like

So, what you should do instead is

$ npm test -- --watch

First pair of -- associates arguments with npm (in our case we don’t need any) and second pair of -- will associate with test command (which is in our case is jest)

Hope you find this info helpful ;)