$ node
> require('bower').commands.install(['https://github.com/examplebrokenurl/doesnotexist.git'], {}, {interactive:false});
{ _interceptors: [], _piped: [] }
> Username for 'https://github.com':

The git child process can show its own interactive prompt at any time (e.g. git interprets 404 on github as authentication error and wants to retry with a password).

To prevent this you can run git with:

 git -c core.askpass=true …

true here is not a boolean value, but a command /bin/true that will make git think it asked for a password.