Edit in JSFiddle

var a = [];

for (var i = 1; i <= 1000; i++)
{ 
  if (i%3 == 0 || i%5 == 0)
  {
    a.push(i);
  }
}

console.log(a);

function sum(a) {
  var total = 0;
  for (var i = 0; i < a.length; i++)
    total += a[i];
  return total;
}

console.log(sum(a));