When I was at Lucidchart, I helped convert the 600k line Closure codebase to TypeScript. [1] In fact, Lucidchart still uses Closure (for minification+library, not typechecking).
There are better approaches available in 2023, but Closure Compiler will always have special place in my heart.
[1] https://www.lucidchart.com/techblog/2017/11/16/converting-60...
Closure brought modules, requires, compile-time type checking and optimizations to JavaScript years before TypeScript was on the scene. I wouldn't dare start a new project with Closure. But it was such a spiritual predecessor to what we have today in TypeScript, and has a special place in my heart too.
The public-facing documentation was terrible, and it got very little evangelism.
Like a lot of Google OSS projects, honestly.
Cool tech, but awkward for everyone not-Google to use.