[wizzwizz4]

At runtime, you can pass primitives to a function that expects a very specific DOM object (say), and it'll just plough on ahead like JavaScript does, until you get an exception that nothing catches because FooFunction doesn't raise any exceptions, and everything breaks.

Oh, and did I mention that virtually every browser API these days is potentially optional, with the undefined primitive replacing bits and pieces of them at seemingly random, depending on browser version and configuration, and the presence or absence of certain extensions…? I like the idea of TypeScript, I really do, but don't even try to use it for FFI (where “foreign” is anything outside your TypeScript module).