It's not "Javascript with a bunch of proprietary extensions", either. Type annotations can't be the difference between understanding code in Typescript if you understand Javascript.
You have a point about tooling, but you would have the same problem if you had a project on visual c++ 6 and upgraded to c++/14, and again, you didn't mention it.
All of this is in spite of the fact that ES6 nearly had optional type annotation in the same syntax that Typescript does, along with many other Javascript derivations that use the same type annotation syntax (Actionscript and Atscript that I know of).
It's not "Javascript with a bunch of proprietary extensions", either. Type annotations can't be the difference between understanding code in Typescript if you understand Javascript.
You have a point about tooling, but you would have the same problem if you had a project on visual c++ 6 and upgraded to c++/14, and again, you didn't mention it.
All of this is in spite of the fact that ES6 nearly had optional type annotation in the same syntax that Typescript does, along with many other Javascript derivations that use the same type annotation syntax (Actionscript and Atscript that I know of).