Improved reduce: Optional Starting Point & Optional Parameters

Lesson

Exercises

function each(coll, f) {
            if (Array.isArray(coll)) {
              for (var i = 0; i < coll.length; i++) {
                f(coll[i], i);
              }
            } else {
              for (var key in coll) {
                f(coll[key], key);
              }
            }
          }
          
          function reduce(array, f, acc) {
            if (acc === undefined) {
              acc = array[0];
              array = array.slice(1);
            }
            each(array, function(element, i) {
              acc = f(acc, element, i);
            });
            return acc;
          }
          

Basic Requirements

1. The factorial function can be written using reduce and range; and furthermore, this is a case where the initial value can be omitted. Write factorial using our updated version of reduce and omit the third argument.

   HINT: Remember that the factorial of a number n is the product of all of the numbers from 1 to n, and the range function (shown below in problem 3) produces an array of numbers from start to end...

2. Use the updated version of reduce to write a function max that computes the maximum number in an array of numbers. Be sure to not include the third argument (the starting point).

3. The range function that we've explored so far looks like this:

   function range(start, end) {
               var acc = [];
               for (var i = start; i < end; i++) {
                 acc.push(i);
               }
               return acc;
             }
             

   Modify range so that the end parameter is optional, and in the event that it is not supplied, range computes a range from 0 to start.

Advanced

There is a way to implement reduce such that it works on both arrays and objects. You'll likely want to make use of Object.keys function.