Type-checking a TypeScript codebase can be slow, especially for monorepos containing lots of projects that each need to use the type checker to generate type declaration files. In this talk, we introduce — for the very first time — a new TypeScript feature we are working on called “Isolated Declarations” that allows DTS files to be generated without using the type checker at all! This opens the door to faster declaration generation in TypeScript itself, as well as in external tools written in other languages such as ESBuild and swc. You'll see how to use this new option, and maybe (just maybe) you’ll be convinced about the benefits of explicit return types! Most importantly, we will show how Isolated Declarations enables parallel builds to spread work across your CPU cores to significantly improve the build speed of your TypeScript projects.

The talk will introduce the concept of variance as in pertains to generic types. It will then show how this concept applies to TypeScript. While showing TypeScript example, I will try to help the audience form an intuition about variance. Finally we will look at what some of the design decisions in TypeScript mean for the soundness of the code we write, and what are some blind spots the compiler has.