As long as we introduced map, we may as well talk about reduce. A lot of people talk about them together. They are two different ways of interacting with data in an array and coming up with a result.

What reduce does is come up with a combined result by performing an operation on each item in the array and adding that value to an accumulator. That continues to pass each time that it iterates through the loop. That accumulator is the value that gets returned by reduce.

var numbers = [1, 2, 3, 5, 8, 13];

var addThem = function(first, second) {
  return first + second;
};

var total = numbers.reduce(addThem);
console.log(total); //32

numbers contains a Fibonacci sequence there. What happens with reduce is it took our function, addThem, and each time that a number came up it added it and used it to add the next value to whatever was in the accumulator from the previous iteration. So first time it added 1 + 2 and what was in the accumulator was 3. Then it added 3 and what was in the accumulator was 6 and so on.

Another convenient thing about reduce is that you can set an initial value for the accumulator. All you have to do is pass that initial value as a second argument when you call reduce.

var numbers = [1, 2, 3, 5, 8, 13];

var addThem = function(first, second) {
  return first + second;
};

var total = numbers.reduce(addThem, 20);
console.log(total); //52

This way we simply start with 20.

reduce also gives you access to the index of the item in the original array. You can even get the original source array. Access these by adding additional arguments to the end of the function that you pass in to your reduce method.

var numbers = [1, 2, 3, 5, 8, 13];

var addThem = function(first, second, count, source) {
  console.log(count);
  return first + second;
};

var total = numbers.reduce(addThem);
console.log(total); //32
// 0
// 1
// 2
// 3
// 4
// 5
// 32

Now our function takes count and source.

So take this new knowledge and consider using forEach, map and reduce the next time you have an array that you want to iterate over. You could reduce your dependence on extraneous variables and also make your code cleaner, more maintainable, and easier to understand.