This video was recorded at Scala Days Berlin 2016 Follow us on Twitter @ScalaDays or visit our website for more information http://scaladays.org Anstract: Have you ever dreamed of understanding how Scala.js works under the covers? Knowing your way around its codebase? This is your chance! This advanced talk will take you to the deepest parts of Scala.js: its compiler, its Intermediate Representation, its linker and its optimizer. Through live coding within the Scala.js codebase, we will explain how the Scala.js compilation pipeline works, and how you can modify it. After this talk, you will understand the Scala.js IR and be able to manipulate it. And you'll be ready to write your custom Linker Plugins! Attendees are expected to be proficient in Scala and have some compiler background (e.g., knowing what an AST is is vital).