The effect is that, if you want to reason about JavaScript programs, you can't carry out your reasoning in JavaScript itself, but rather have to use an external metalanguage - ideally something like Hoare logic, but, realistically, usually an informal, insufficiently understood, even insufficiently defined language, like English.
OTOH, a Haskeller can reason about Haskell programs by just evaluating Haskell expressions.
OTOH, a Haskeller can reason about Haskell programs by just evaluating Haskell expressions.