My codes folders and files like this, you never know how many sub folders in it:
js/sub1/a.js js/sub2/b.js js/sub3/sub31/c.js js/sub4/sub41/sub411/d.js >
js/sub1/a.js js/sub2/b.js js/sub3/sub31/c.js js/sub4/sub41/sub411/d.js
Per the official documentation on file globbing, to watch for changes for files of a certain file type in the directory path and its subdirectories, you'll want:
files: ['js/**/*.js']