It is a full static checking system, so it will find obvious errors like mistyping something, but it also infers alot of information from the context.

I made a small simple demo here: https://www.typescriptlang.org/play/index.html#src=const%20e...

Note: Click Options and enable all the settings. Typescript works best in strict mode, but it seems options arn't including in sharing via url.

it'll throw an error if a parameter is undefined, and https://github.com/Microsoft/TypeScript/issues/15333 which lands in the next version auto-suggests spelling corrections as well

Of course. That's table stakes for any static type checking.

In fact to set a global value (ex: window.Foo) requires a bit of extra work to instruct the type check that your new property is not errant.

What is the error in 1+'1'? It seems like it would just be '11'.

Unless, of course, you're implying it converts javascript into a strongly typed language, which would be amazing.