// Conditional types (runtime illustration) type IsString<T> = T extends string ? true : false; type ArrayElement<T> = T extends (infer U)[] ? U : never; const arr = [1, 2, 3]; console.log(typeof arr[0]); // 'number' function isString(x: unknown): boolean { return typeof x === "string"; } console.log(isString("hello")); // true console.log(isString(42)); // false