I've never worked for Google, but have written a lot of Closure code. Unfortunately, my opinions about Closure are colored by how it was used within my org --

I've never worked for Google, but have written a lot of Closure code. Unfortunately, my opinions about Closure are colored by how it was used within my org -- which was far from ideal. We should have taken the best of Closure (the compiler, type checking, and maybe a few utility libraries) and ignored most of the rest. When we switched all greenfield JS development from Closure to React around 2016, it resulted in a 2-3x speed-up in the development process. Not because the compiler stage added any significant time to our process, but because of poor design patterns that were implemented because Google was using them -- but perhaps not with the correct level of understanding about how to use them effectively.