Narrow types when using `Array.filter`

[NoelAbrahams]

Hi,

Will this be possible?

var foo: (number|string)[] = [1, 'bar'];

foo.filter(item => typeof item ==='string').map(item => item/*string type*/);
foo.filter(item => typeof item !=='string').map(item => item/*number type*/);

[RyanCavanaugh]

Not happening unless someone can figure out a reasonable way to write a signature for filter in a way that is generalizable.