Confused about Type Predicate/Guard Safety

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In this tweet Cory recommends a User Defined Type Guard as a safer alternative to a type assertion/cast

e.g. instead of fish as Fish

function isFish(pet: Fish : Bird): pet is Fish {
    return (pet as Fish).swim !== undefined;
}

this does look 'safer' to me.. sure it's basically still doing the cast but it checks it has the swim method first, so..

why does matt say this is a "common mistake" (link to tweet)

What am i missing?

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Posted: 8 months ago
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