block2.ts

const arr: Array<string | number> = [1, 'a', 2];

arr.forEach((value: string | number) => {
    if (typeof value === 'number') {
       //Here `value` is known to be a number by the compiler
        console.log(value * 2);
    } else {
       //Here `value` is known to be a string by the compiler
       console.log(value.substr(1));
    }
});