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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TS can do this with user-defined type guards: https://www.typescriptlang.org/docs/handbook/advanced-types.html …
Same as Flow's %checks then. It's not a very ergonomic way to define refinements, and you're limited to the simple syntax that it lets you define.
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It's actually more flexible because you could write a runtime check for a very complex object by using duck typing and then add the `is` return type. However, it would be nice if TS just figured out the single typeof automatically like you mentioned.
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TypeScript would then just trust that your type guard was accurate then. In a different model, by the time you returned a boolean in the type guard function, you can save the refinements made to the input variable. That way you have more safety.
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