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When using Flow, be careful with empty objects. They are "unsealed" which means that all properties are effectively optional at the original call site. Meaning, this does NOT error: const obj: {foo: string} = {}; This behaviour is specific to an empty object.

So this will pass flow check but throw at runtime? type Foo = { baz: () => void } function callBaz(foo: Foo) { // baz exists because types return foo.baz() } const foo: Foo = {} callBaz(foo) > UNDEFINED IS NOT A FUNCTION