Native vs. selfwritten js code

var benchmark = function(func, label) {
  label = label || "benchmark"
  console.time(label)
  func.call(null)
  console.timeEnd(label)
}
var arr = []
var isEven = function(elem) {
  return elem % 2 == 0
}

for(var i = 0; i < 10000000; i++) {
  arr.push(i)
}

Array.prototype.filterCustom = function(callback) {
  var filtered = []
  for(var i = 0; i < this.length; i++) {
    if(callback(this[i])) filtered.push(this[i])
  }
  return filtered
}

benchmark(function() { arr.filterCustom(isEven) }, 'custom filter function')
benchmark(function() { arr.filter(isEven) }, 'native filter function')

the custom filter function is much faster than the native one...

tested in node and chrome

and what was the result?