Javascript Array map(grade)

var school = [1, 2, 3, 4, 5, 6, 7, 8, 3, 35, 3, 5];

Array.prototype.map = function(grade) {
  var result = [];
  this.forEach(function(item) {
    result.push(grade(item));/*from ww  w  .j  av  a2  s  .  c o m*/
  });
  return result;
};

var a = school.map(function(x) {
  return x + 12;
});

console.log(a);