he/him 
Actually TypeScript errors on the second one!
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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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Further, this should also be a warning: const nums = [1, 2, 3]; const num: number = nums[4]; // impossible, no bounds check
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For arrays of unknown length, the type should be `?T`. You can support refinements of `arr.length` to avoid a majority of these.
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As far as I know, very few type systems of any language do the dependent type gymnastics to treat that as a specifi literal rather than just “some string”
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