fun.js

//The fun solution: using array methods
var ary = [];
for(var i = 1; i <= 1000; i++){ ary.push(i); }
console.log(
  ary.filter(function(item){
    return (item % 3 == 0 || item % 5 == 0)
  }).reduce(function(memo, current){ 
    return memo + current
  }, 0)
);

obvious.js

//The obvious solution: no array magic stuff
console.log((function the_sum(min, max){
  var sum = 0; 
  for(var i= min; i <= max; i++){
    if(i % 5 == 0 || i % 3 == 0){
      sum += i
    }
  } 
  return sum
})(1,1000))