The post covers this in detail. JavaScript at Google was almost completely different from JavaScript elsewhere. The tools were different, the libraries are different, and even the language itself is different (jscompiler’s ES4-inspired type system).

It’s also a huge company with a gigantic monorepo (there’s papers about it elsewhere) and a lot of inertia.

It’s amazing what Evan and his team were able to do for front-end development at Google by building tools to bridge between Google JS and TypeScript.