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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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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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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End of conversation
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It's a it of duplication, but you can get the errors with typescript today if you enforce it's length const nums: [1, 2, 3] = [1, 2, 3]; const num: number = nums[4]; // fails And it's been marked for revisiting to have this behaviour by default
https://github.com/Microsoft/TypeScript/issues/24350 …Thanks. Twitter will use this to make your timeline better. UndoUndo
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Definitely, that would make Typescript way safer ! Also, a pipe operator and optional chaining would be great
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