Truthy check loses effect based on position in "&&" chain

I have three checks in a "&&" chain. The order of the first two checks seems to have an effect on whether I can perform the third check. Am I missing something?

function f1(x: unknown): any {
    return x && typeof x === 'object' && x.hasOwnProperty('x');
    //                                   ^  TS reports error: Object is possibly 'null'
}

function f2(x: unknown): any {
    return typeof x === 'object' && x && x.hasOwnProperty('x');  // no error
}

Reproduced in 3.8-beta (playground link)

For you to safely call hasOwnProperty on x, x needs to be of type object, and it needs to be non-null.

In f2: After narrowing the type of x using typeof x === 'object', its type is object | null. Narrowing object | null using trueish x && it becomes object.

In f1: However, narrowing unknown with x &&, it is still unknown. Therefor it only has type object | null when you try to call x.hasOwnProperty().

You can see the type by hovering x at the different positions in your playground links.

In the absence of negated types (which themselves have issues) we have no accurate representation of truthy values in the type system. So, an unknown narrowed by a truthiness check is still an unknown. We could potentially consider narrowing to {}, which at least eliminates null and undefined, but {} is an odd type and it doesn't eliminate the other falsy values.

Alternatively, we could special case the pattern of x && typeof x === 'object' and just narrow to object in that case. That's probably the most realistic solution.

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Truthy check loses effect based on position in "&&" chain #36870

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Truthy check loses effect based on position in "&&" chain #36870

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markusjohnsson commented on Feb 19, 2020

ahejlsberg commented on Mar 16, 2020

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