SebastianVerified account

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

Further, this should also be a warning: const nums = [1, 2, 3]; const num: number = nums[4]; // impossible, no bounds check

For arrays of unknown length, the type should be `?T`. You can support refinements of `arr.length` to avoid a majority of these.

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 }