in Operator for Narrowing Section

[karlhorky]

Hi, thanks for this awesome new handbook, I like the new way that you're presenting TypeScript here!

Type guards with the in operator are not mentioned in the Narrowing section.

They are mentioned in the current handbook on the Advanced Types page under the Using the in Operator section:

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Using the in operator

The in operator now acts as a narrowing expression for types.

For a n in x expression, where n is a string literal or string literal type and x is a union type, the “true” branch narrows to types which have an optional or required property n, and the “false” branch narrows to types which have an optional or missing property n.

function move(pet: Fish | Bird) {
    if ("swim" in pet) {
        return pet.swim();
    }
    return pet.fly();
}

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Potentially related to #9 by @gcrev93