1. Write a function called each that takes in an array and a callback function. each should iterate through all items in the array and call the callback function with each item and its index as parameters. each should return the original array that was passed in.

   // `each` takes in an array and a callback function
   var each = function(list, callback) {
    // iterates through each item in array
    for (var i = 0; i < list.length; i += 1) {
      // calls callback function with item and index
      callback(list[i], i);
    }
    // returns original array
    return list;
   };
   

2. Write a function called map that takes in an array and a callback function. map should iterate through all items in the array, call the callback function with each item and its index as parameters, and return a new array of the results.

    // `map` takes in an array and a callback function
    var map = function(list, callback) {
      var mappedList = [];
      // iterates through each item in array
      for (var i = 0; i < list.length; i += 1) {
        // calls callback function with item and index (adds result to `mappedList`)
        mappedList.push(callback(list[i], i));
      }
      // returns mapped array of results
      return mappedList;
    };
   

3. Write a function called reduce that takes in an array of numbers. reduce should return the sum of all the numbers in the array.

  // `reduce` takes in an array of numbers
  var reduce = function(numList) {
    var sum = 0;
    for (var i = 0; i < numList.length; i += 1) {
      sum += numList[i];
    }
    return sum;
  };