How to add 2 adjacent numbers in 1 line, with the condition that they are the same?

Question

JavaScript

How to add 2 adjacent numbers in 1 line, with the condition that they are the same. For example, the line 111111 should turn into line 222 and then into 42.

Answer 1

function collapse(digits) {
    if (typeof digits !== 'string') {
        digits = '' + digits;
    }

    var result = '';

    for (var i = 0; i < digits.length; i++) {
        if (digits[i] === digits[i+1]) {
            result += (parseInt(digits[i]) + parseInt(digits[i+1]));
            i++;
        } else {
            result += digits[i];
        }
    }

    if (digits.length === result.length) {
        return result;
    } else {
        return collapse(result);
    }
}

var test = 111111;

console.log(collapse(test));

Answer 2

function collapse(src) {
  var res = String.prototype.replace.call(src, /(\d)\1/g, (s, p) => p * 2);
  return res == src ? res : collapse(res)
}