"Typical" solution for You Can't JavaScript under pressure Step 5

gistfile1.js

function monkey(arr) {
    var len = arr.length,
        cur,
        finalVal = 0,
        i;

    for (i = 0; i < len; i++) {
        cur = arr[i];
        if (cur instanceof Array) {
            finalVal += monkey(cur);
        } else if (typeof cur === 'number') {
            finalVal += cur;
        }
    }

    return finalVal;
}