checkArrayAndSum

var arr = [1, 2, 3];

function validateArr(arr){ // validate if items contains only numbers
    var i = 0;
    var l = arr.length;
    var areNumber = true;

    for (i; i< l; i++){   // check if ALL items are numbers, for every "success check" counter ++
        counter = 0;
        if(isNaN(arr[i])){
            areNumber = false;
        }
        else{
            counter++;
        }
    }
    if(counter === l){    // here check if the counter value are equal to length of the array
        areNumber = true;
    }

    return areNumber;
}

function sumItems(arr){   // here we sum every item of the array
    var i = 0;
    var l = arr.length;
    var result = 0;
    for (i; i< l; i++){
        result = result + arr[i];
    }
    return result;
}

if(validateArr(arr) == true){ // output the result if everything is fine
    console.log(sumItems(arr));
}
else{
    console.log("error");
}