en.javascript.info/1-js/4-data-structures/10-arguments-pseudoarray/2-sum-arguments/solution.md

//+ run
function sum() {
  var result = 0;

  for (var i = 0; i < arguments.length; i++) {
    result += arguments[i];
  }

  return result;
}

alert( sum() ); // 0
alert( sum(1) ); // 1
alert( sum(1, 2) ); // 3
alert( sum(1, 2, 3) ); // 6
alert( sum(1, 2, 3, 4) ); // 10