> ...if you e.g. change the type of a function, you have to change or comment out all the code that calls that function before it'll compile. I thought so for

> ...if you e.g. change the type of a function, you have to change or comment out all the code that calls that function before it'll compile. I thought so for a long time too, but then I discovered `undefined`. You can replace any expression with undefined and if there's a type, it will compile, and explode at runtime when it tries to evaluate it.