[рони]

Cuntmann,

function f(ar) {
            let i = 0;
            for (let n of ar.slice(1)) {
                if (typeof (n + ar[i]) == 'number') ar[i] += n;
                else ar[++i] = n;
            }
            ar.length = ++i;
            return ar.join('');
        }

        console.log(f([1, 1, 'Q', 1, 'R'])); //2Q1R
        console.log(f([1, 1, 1, 'Q', 1, 1, 'R', 1])); // 3Q2R1
        console.log(f([1, 1, 'N', 'W', 1, 1, 1, 'Q'])); // 2NW3Q
        /*****/
        console.log(f([1, 11, 'N', 'W', 3, 55, 0, 'Q'])); // 12NW58Q
        console.log(f(['A', 1, 21, 'N', 'W', 3, 55, 0, 'Q', 4])); // A22NW58Q4
        console.log(f(['A', 'CDE', 1, 21, 'N', 'W', 3, 'Q', 4])); // ACDE22NW3Q4