The example starting at 17:00 says:
Imagine that you have a 200 lines object in the return statement. If you mistype or make an error, JS tells you nothing about it, so you have to stare at the 200 lines of code to find the error. TS will help you find error.
Yeah, but how? When in the example "add" is mistyped to "addx", the whole return statement (only 3 lines in this case) was underlined indicating an error, not just the specific line containing an error.
Anyway, TS looks great, I'll definitely try it!