var main = [1, 2, 3, "a", "b", 7, 8, "f"]
for(var i = 0, len = main.length; i < len; i++) {
    if(typeof main[i] == 'number') {
        main[i] = main[i]*2
    } else if(typeof main[i] == 'string') {
        main[i] = main[i] + main[i]
    }
}