calculator.js

/*

String Calculator
 
    1- The program can take 0, 1 or 2 numbers and will
    return their sum, (for an empty string it will return 0)
    for example "" or "1" or "1,2" -> 0, 1, 3
 
    2- Allow the Add method to handle an unknown amount
    of numbers
 
    3- Allow the Add method to handle new lines between
    numbers(instead of commas). The following input
    is ok: "1\n2,3" (will equal 6)
 
    4- Support different delimiters. To change a delimiter
    the beginning of the string will contain a separate
    line that looks like this: "//[delimiter]\n[numbers...]"
    for example "//;\n1;2" should return 3 where the
    default delimiter is ";"

*/

var add = function(str){
    var delimiters = [",","\n"];

    if (str == ""){
        return 0;
    }

    if (str.indexOf("//") == 0){
        delimiters.push(str[2]); // add new delimiter
        str = str.substr(4);     // ignore first 5 characters
    }

    var numbers = str.split(
        new RegExp(delimiters.join("|"))
    );

    return numbers.reduce(function(total,n){
         return total + parseInt(n);
    },0); 
}

test_calculator.js

// http://jsfiddle.net/ZAJt4/
test("test_should_return_zero_on_empty_string", function() {
  equal(add(""),0);
});

test("test_should_return_self_on_number", function() {
  equal(add("1"),1);
  equal(add("2"),2);
});

test("test_should_return_sum_on_two_numbers", function() {
  equal(add("1,2"),3);
  equal(add("3,3"),6);
});

test("test_should_return_sum_on_two_numbers", function() {
  equal(add("1,2,3"),6);
  equal(add("1,2,3,4"),10);
});

test("test_should_return_sum_on_two_numbers", function() {
  equal(add("1\n2"),3);
  equal(add("1\n2,3"),6);
});

test("test_should_accept_custom_delimiters", function() {
  equal(add("//;\n1;2"),3);
  equal(add("//;\n1;2\n3"),6);
});