It's exactly like TypeScript. You write type definitions or you find them online, like in TypeScript, then you can call any JavaScript API using the relevant types.

Full relevant docs at https://www.scala-js.org/doc/interoperability/

Many TypeScript type definitions are also automatically converted to ScalaJS: https://github.com/oyvindberg/ScalablyTyped

You can call js directly using dynamic. From the docs[1]:

    val document = js.Dynamic.global.document
    val playground = document.getElementById("playground")
    val newP = document.createElement("p")
    newP.innerHTML = "Hello world! <i>-- DOM</i>"
    playground.appendChild(newP)

1. http://ochrons.github.io/sjs2/doc/interoperability/calling_j....