TypeScript/tests/baselines/reference/isArray.js at 8f908812977f5d0e4b910f8ec7cbcb216c0f7693 · microsoft/TypeScript · GitHub

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//// [isArray.ts]
/// @errors: 2322 4104

// https://github.com/microsoft/TypeScript/issues/17002
// Preserves mutability, false branch removes arrays, mutable or not

declare const mutable: string | string[];
if (Array.isArray(mutable)) {
  const stillMutable: string[] = mutable;
} else {
  const narrowed: string = mutable;
}

declare const immutable: string | readonly string[];
if (Array.isArray(immutable)) {
  const notMutable: string[] = immutable; // Should fail: readonly string[] isn't assignable to string[]
} else {
  const narrowed: string = immutable;
}

// https://github.com/microsoft/TypeScript/issues/33700
// Preserves element or iterated type of wider types

declare const arrayLike: string | ArrayLike<string>;
if (Array.isArray(arrayLike)) {
  const arrayOfElementType: readonly string[] = arrayLike;
  const notArrayOfAny: readonly void[] = arrayLike; // Should fail: string isn't assignable to void
}

declare const iterable: string | Iterable<string>;
if (Array.isArray(iterable)) {
  const arrayOfIteratedType: readonly string[] = iterable;
  const notArrayOfAny: readonly void[] = iterable; // Should fail: string isn't assignable to void
}

// https://github.com/microsoft/TypeScript/pull/42316#discussion_r823218462
// any and unknown backward compatibility

declare const any: any;
if (Array.isArray(any)) {
  const mutableArrayOfAny: void[] = any;
  const notAny: void = any; // Should fail: any[] isn't assignable to void
}

declare const unknown: unknown;
if (Array.isArray(unknown)) {
  const mutableArrayOfAny: void[] = unknown;
  const notAny: void = unknown; // Should fail: any[] isn't assignable to void
}


//// [isArray.js]
/// @errors: 2322 4104
if (Array.isArray(mutable)) {
    const stillMutable = mutable;
}
else {
    const narrowed = mutable;
}
if (Array.isArray(immutable)) {
    const notMutable = immutable; // Should fail: readonly string[] isn't assignable to string[]
}
else {
    const narrowed = immutable;
}
if (Array.isArray(arrayLike)) {
    const arrayOfElementType = arrayLike;
    const notArrayOfAny = arrayLike; // Should fail: string isn't assignable to void
}
if (Array.isArray(iterable)) {
    const arrayOfIteratedType = iterable;
    const notArrayOfAny = iterable; // Should fail: string isn't assignable to void
}
if (Array.isArray(any)) {
    const mutableArrayOfAny = any;
    const notAny = any; // Should fail: any[] isn't assignable to void
}
if (Array.isArray(unknown)) {
    const mutableArrayOfAny = unknown;
    const notAny = unknown; // Should fail: any[] isn't assignable to void
}