On: All I want for Christmas is these seven TypeScript improvements

[akdor1154]

    [1,2,undefined,3]
    .filter(<T>(x: T|undefined): x is T =>
        typeof x !== 'undefined'
    );

Yes builtin lang support would be better, but that gets you out of trouble in a pinch, without adding more 5 line npm baggage.

N.B. if you are in a JSX file, you'll need to add a comma: `<T,>`