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Array.isArray works bad with ReadonlyArray #43247
Description
TypeScript Version: 4.3.0-dev.20210315
Search Terms: array isArray readonly
The first problem is that Array.isArray lets us violate readonly modifier and push values to ReadonlyArray:
declare const data: ReadonlyArray<any>
if (Array.isArray(data)) {
// Expected: Error
data.push(123)
}The second problem is that Array.isArray doesn't filter out ReadonlyArray type:
declare const data: Record<string, any> | ReadonlyArray<any>
if (!Array.isArray(data)) {
// now: readonly any[] | Record<string, any>
// expected: Record<string, any>
const alias = data
}
Activity
MartinJohns commentedon Mar 15, 2021
This is most likely intentional. Be aware that readonly does not mean immutable. There is no way to represent immutability in TypeScripts type-system.
See #17002. Found using your search terms.
RyanCavanaugh commentedon Mar 15, 2021
This is the intended behavior. A regular array is a subtype of a ReadonlyArray (otherwise this hierarchy would not be useful). And if a type guard can't narrow down a type to a subtype (otherwise a type guard would not be useful).
typescript-bot commentedon Mar 17, 2021
This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.