RxJS is a library for composing asynchronous and event-based programs by using observable sequences. 


rxjs

JavaScript® (often shortened to JS) is a lightweight, interpreted, object-oriented language with first-class functions, most known as the scripting language for Web pages, but used in many non-browser environments as well such as node.js or Apache CouchDB. It is a prototype-based, multi-paradigm scripting language that is dynamic, and supports object-oriented, imperative, and functional programming styles.

javascript

***Note: Some of the RxJS syntax used in this course is outdated, but the concepts taught on arrays, async, and observables are still reliable and applicable.***

Async programming may seem daunting. How can we write programs that accept user input, run animations, and send server requests over the same period? How do we keep the code clear and concise? How do we gracefully propagate and handle asynchronous errors? How can we avoid memory leaks caused by dangling event handlers? JavaScript's loops and try/catch keywords are no help - they only work on synchronous functions.

Here's the good news: Asynchronous programming is much easier than it seems. The key is to think differently about events. It is possible to build most asynchronous programs using a handful of simple functions. We will learn why most JavaScript developers approach asynchronous programming the wrong way, and how to avoid these common mistakes. 

Maxime Bourgeois

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.

Create an Array concatAll method

In this lesson we will get introduced to the `Observable` type. An Observable is a 'collection that arrives over time'. Observables can be used to model events, asynchronous requests, and animations. Observables can also be transformed, combined, and consumed using the Array methods we learned in the previous lessons. We can write powerful and expressive asynchronous programs using the few simple methods we've learned so far.

Introducing the Observable

Like an array, Observable has a map method that allows us to transform a sequence into a new Observable.

Using the map method with Observable

Armed with the `map` and `concatAll` functions, we can create fairly complex interactions in a simple way. We will use Observable to create a simple drag and drop example with basic DOM elements.

Simple drag and drop with Observables

In this lesson we solidify our understanding of how to flatten collections. This is perhaps the most important skill when learning to program without loops. We will try our hand at flattening not just a two dimensional collection, but a three-dimensional collection. Later on it will become clear how these skills relate to asynchronous programming.

Asynchronous Programming: The End of The Loop

Jafar Husain

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