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There is a way to transform a typescript definition to Scala.js facade (https://github.com/sjrd/scala-js-ts-importer).
As for the bloat, it isn't much. Compiled Javascript for our full-fledged app is about 180KB (gzipped) + external libraries such as React.
In our case, the production JS file produced by Scala.js is about 180KB (gzipped). This size does not include third-party code such as React and other assets.
We are building a production app using Scala.js (with React as the underlying framework through https://japgolly.github.io/scalajs-react). It has been a marvelous and super-productive experience.
Here are the slides from my recent presentation at "Scala by the Bay" conference narrating our experience so far (video for the same will be released by the conference organizers soon): http://www.slideshare.net/ramnivas2/full-stack-scala