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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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.
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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Good point
Maybe @drosenwasser or@SeaRyanC could explain why type guards only narrow down the type in an `if` statement and not in a boolean function. -
Automatically inferring functions as type predicates - could break code - is not always the intent - is detectable for some simple cases, but can be expensive for the hard ones so it might not be consistent To be honest I get why Flow also went with opt-in.
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