How to type reduce Typescript?

Question

typescript

Tell me what TS scolds and how to fix it? Sandbox

Answer 1

const res = arr.reduce((acc, cur) => acc  + cur.counter, 0);

Answer 2

interface Counter {
   counter: number;
}

const arr: Counter[] = [{counter:120}, {counter:220}];

const res = arr.reduce((acc: number, cur: Counter) => {
    return acc  + cur.counter;
}, 0);