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Truthy check loses effect based on position in "&&" chain #36870
Description
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)
Activity
markusjohnsson commentedon Feb 19, 2020
For you to safely call
hasOwnPropertyonx,xneeds to be of typeobject, and it needs to be non-null.In f2: After narrowing the type of
xusingtypeof x === 'object', its type isobject | null. Narrowingobject | nullusing trueishx &&it becomesobject.In f1: However, narrowing
unknownwithx &&, it is stillunknown. Therefor it only has typeobject | nullwhen you try to callx.hasOwnProperty().You can see the type by hovering
xat the different positions in your playground links.ahejlsberg commentedon Mar 16, 2020
In the absence of negated types (which themselves have issues) we have no accurate representation of truthy values in the type system. So, an
unknownnarrowed by a truthiness check is still anunknown. We could potentially consider narrowing to{}, which at least eliminatesnullandundefined, 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 toobjectin that case. That's probably the most realistic solution.