Number

[meriah]

let num = prompt('Введите число')
for (let i = 0; i < num.length; i++) {

x = Number(num[i]) + Number(num[i + 1])

alert(Сумма всех цифр числа: ${x})

}

Here's my guess, but in my case, when all the numbers are the same, the first two digits are summarised.

[elysha]

function fSumAllDigits(n) {
  return [...n.toString()].reduce((acc, el) => acc + parseInt(el), 0);
}
console.log( fSumAllDigits(111) ); // 3
console.log( fSumAllDigits('123') ); // 6
console.log( fSumAllDigits(700000001) ); // 8