Get to know Javascript's Array. Learn to harness it. Learn to embrace it. Learn to love it.

Let's take a closer look at using Javascript's built in Array reduce function. Reduce is deceptively simple and when harnessed correctly can achieve very powerful results. By leveraging reduce, we can answer a variety of questions on a single, simple data set. In this lesson, we'll look at how you might use [Array.prototype.reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce) to:

- Sum an array of numbers
- Reduce an array of objects to a sum of a given property
- Group an array of objects by key or a set of given criteria
- Count the number of objects in an array by key or a given set of criteria

Array.prototype.reduce in JavaScript by example

Refactoring is a great opportunity to learn and we have an existing Javascript function that is *lengthy, hard to understand, and overcomplicated*. We'll take a look at putting some of **Javascript's built in Array.prototype methods** to use to help us clean this up and learn by example through refactoring.

By taking a few minutes to refactor this existing code, we can get a glimpse at how we can harness the power of the Array and lean on some built in Array functions available to us in Javascript's core without any added utility libraries.

In this lesson we touch on just a few of the [Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) methods:

- [Array.prototype.forEach](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach)
- [Array.prototype.filter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter)
- [Array.prototype.some](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some).

Refactoring: Array.prototype by example [filter, some, forEach]

Concat creates a shallow copy of an existing array that includes any arguments you pass to it. In this lesson, we look at using concat for adding additional values to an array then cover some more useful features such as accepting other arrays as arguments & how to chain concat with other array methods such as `forEach`

Use Concat to Add Values to an Array

Array Comprehensions didn't make the ES6 cut. You probably shouldn't use them ;)

See comments.

Array Comprehensions - NON-STANDARD

Most JavaScript developers are familiar with the `for` loop. One of the most common uses of the `for` loop is to iterate through the items in an array. In this lesson, we will learn how to replace the `for` loop with the Array's `forEach` method - and shorten your code in the process.

The Array forEach method

One very common operation in programming is to iterate through an Array's contents, apply a function to each item, and create a new array containing the results. For example, let's say you wanted to loop through an array of stock objects and select only the name for display on screen. In this lesson we will demonstrate how to use the Array's `map` method to easily perform this operation with less code than a loop would require.

The Array map method

One very common operation in programming is to iterate through an Array's contents, apply a test function to each item, and create a new array containing only those items the passed the test. For example, let's say you wanted to loop through an array of stocks and select only those with the price larger than a certain value. In this lesson we will demonstrate how to use the Array's `filter` method to easily perform this operation with less code than a loop would require.

The Array filter method

Both `map` and `filter` do not modify the array. Instead they return a new array of the results. Because both `map` and `filter` return Arrays, we can chain these functions together to build complex array transformations with very little code. Finally we can consume the newly created array using `forEach`. In this lesson, we will learn how to build nontrivial programs without using any loops at all.

Chaining the Array map and filter methods

In addition to flat Arrays, programmers must often deal with nested Arrays. For example let's say we have an Array of stock exchanges, each of which is represented by an array of all the stocks listed on that exchange. If we were looking for a stock that matched a certain criteria, we would first need to loop through all of the exchanges, and then all of the stocks within. 

In these situations, most developers would nest two loops. However in this lesson we will write a new Array function `concatAll` which will automatically flatten nested arrays buy one dimension. This will remove the need to ever use a nested loop to flatten a nested array.

Array Goodness

Instructor

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