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fix(lib): preserve readonly narrowing in Array.isArray type guard #63609

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fix(lib): preserve readonly narrowing in Array.isArray type guard #63609

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1 change: 1 addition & 0 deletions src/lib/es5.d.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1499,6 +1499,7 @@ interface ArrayConstructor {
(arrayLength?: number): any[];
<T>(arrayLength: number): T[];
<T>(...items: T[]): T[];
isArray(arg: readonly any[] | any): arg is readonly any[];
isArray(arg: any): arg is any[];
readonly prototype: any[];
}
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26 changes: 26 additions & 0 deletions tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,26 @@
// @strict: true

function test1(x: readonly string[] | string) {
if (Array.isArray(x)) {
x; // should be readonly string[]
}
}

function test2(x: readonly number[] | number) {
if (Array.isArray(x)) {
x; // should be readonly number[]
x[0]; // should be number
}
}

function test3(x: string[] | string) {
if (Array.isArray(x)) {
x; // should still be string[] (not readonly)
}
}

function test4(x: unknown) {
if (Array.isArray(x)) {
x; // should still be any[] (existing behavior)
}
}