Because Magenta.js sits in the browser, it is a bit harder to make it interact with other applications such as a DAW than a Magenta application, but as web standards evolve, this will become easier.
Making Magenta.js interact with other apps
Using the Web MIDI API
The Web MIDI API (www.w3.org/TR/webmidi/) is a W3C standard with a specification that isn't very mature, with the status of W3C Working Draft March 17, 2015. It isn't well supported across browsers, with Firefox and Edge having no support at all. It works pretty well in Chrome, though, so if you require your users to use that browser, your application might work. See the last section, Further reading, for more information.
You can find this code in the...