he/him 
No JS tool (that I know of) will warn you about these conditions being impossible. But they should. const foo = 'FOO'; if (foo[0] === 'B') {} // impossible if (foo === 'BAR') {} // impossible
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Also refinements should work with function calls: function isString(str) { return typeof str === 'string'; } declare var foo: string | number; if (isString(foo)) { foo; // foo has been refined to string here } else { foo; // foo has been refined to number here }
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And it should work across files, and shouldn't require a %checks annotation like in Flow.
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I can't get that notation to work on a function :/
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