LX) ‘
declare function assert(value: unknown): asserts value;
declare function assertIsArrayOfStrings(obj: unknown): asserts obj is string[];
declare function assertNonNull<T>(obj: T): asserts obj is NonNullable<T>;
function f1(x: unknown) {
assert(typeof x === "string");
return x.length; // x has type string here
!
function f2(x: unknown) {
assertIsArray0fStrings(x);
return x[0].length; // x has type string[] here
}
function f3(x: string | undefined) {
assertNonNull(x);
return x.length; // x has type string here
h