Hundreds of programming languages compile to JavaScript. Yet, most of them fail, at one level or another, to provide satisfactory interoperability with JavaScript APIs. This is limiting, as interoperability is at least required to manipulate web pages through the DOM API, but also to use the ecosystem of existing JavaScript libraries. This paper presents the interoperability features of Scala.js, which solves the shortcomings of previous approaches. Scala. js offers a separate hierarchy of JavaScript types, whose operations have semantics borrowed from ECMAScript 2015. The interoperability features are complete with respect to ECMAScript 2015, save for two exceptions which are still being worked on. This allows Scala. js programs to perform any operation that an ECMAScript program could do, thereby guaranteeing that they can talk to any JavaScript library.