Creating a new GridStore instance, id and filename are listed as optional parameters.

var grid = new GridStore(db, 'w');

Produces
{{TypeError: Cannot read property 'indexOf' of undefined
at new GridStore (.../node_modules/mongodb/lib/gridfs/grid_store.js:107:13)}}

It looks like, after it checks that id is not an ObjectID and that filename is undefined, it never assigns mode = id; to accommodate the id parameter being optional.

I'm able to sidestep the issue by including an arbitrary placeholder for id such as var grid = new GridStore(db, null, 'w'); or var grid = new GridStore(db, 'anything thats not an objectID', 'w');