text: function last(arr) { return arr[arr.length - 1]; }
  
  last([1, 2, 3]); //: number
  last(["a", "b", "c"]); //: string
  
  function map(arr, f) {
    var res = [];
    for (var i = 0; i < arr.length; ++i) res.push(f(arr[i]));
    return res;
  }
  
  map([1, 2, 3], function() { return "X"; }); //: [string]
  map([1, 2, 3], function() { return true; }); //: [bool]
  