Reactive programming

Reactive programming is getting a lot of focus lately. This idea is relatively new and, like many new ideas, has lots of confusing, and sometimes contradictory information floating around. We discussed asynchronous programming earlier in this book. JavaScript takes asynchronous programming to new heights by providing first class language constructs that support it.

Reactive programming is essentially programming with asynchronous event streams. An event stream is a sequence of events happening over time. Consider the following diagram:

In the preceding diagram, time passes from left to right and different events occur over time. As the event happens over time, we can add an event listener to this whole sequence. Whenever an event happens, we can react to it by doing something.

Another type of sequence in JavaScript is an array. For example, consider the following lines of code:

    var arr = [1,1,13,'Rx',0,0]; 
    console.log(arr); 
    >>> [1, 1...