gistfile1.js

function isArray(obj) {
  return Object.prototype.toString.call(obj) === "[object Array]";
}

function isFunction(obj) {
  return Object.prototype.toString.call(obj) === "[object Function]";
}

function forEach(list, callback) {
  if (isArray(list)) {
    for (var n = 0; n < list.length; n++) {
      if (isFunction(callback)) {
        callback.call(list[n], n);
      } else {
        // it fails
        assert(isFunction(callback) === true, "[Info] This callback is not a function :(");
      }
    }
  } else {
    // it fails
    assert(isArray(list) === true, "[Info] This list is not a array :(");
  }
}

window.onload = function() {
  var weapons = ["shuriken", "katana", "nunchucks"];
  forEach(weapons, function(index) {
      assert(this == weapons[index], weapons[index] + " is expected value");
  });

  // not Array
  var meal = "onigiri";
  forEach(meal, function(index) {
      assert(this == meal,  "this is not called");
  });

  // not Function
  var towns = ["iga", "koga", "uraga"];
  forEach(towns, null);
};