Introduction to react.js

React is a JavaScript framework built at Facebook, it was built to answer the question "How should we structure JavaScript applications".
There are a lot of JavaScript frameworks that try to answer this question, most of them are MVC based (or MVVM or MVW) - basically they're all based around models - which are just observable objects that have some events api that allows you to subscribe to some changes on that object. So developers set up bi-directional data-binding that allow you to subscribe to changes on you r model, so whenever something changes you can mutate and update your view.

React is a JavaScript library for building user interfaces, you get all the good parts of a complete render, but without the downsides such as performance and loss of data.

At the heart of react is, declarative components - describing what components look at at any point in time

Initial Render

There is no explicit data binding, in react we just define one render function, and the purpose of this render function is to describe what your view looks like in any point in time. It returns a representation of your view. We recursive call render to build up this hierarchy. When we want to generate the mark-up of this representation for the first time, we take the representation and iterate over it generate a string and inject it into the document. This does something called two-pass rendering which is generating the string, then later, after the string is injected into the document we attach the event handlers at the top-level, which exposes some really interesting opportunities, since your generating your string somewhere separate from where your hooking up your events, you can render on the server.

Update Rendering

Instead of mutation for updating react uses a process called reconciliation, the purpose of this is to keep you UI up-to-date as your data changes, automatically updates your views and DOM. The render function that does the initial rendering and returns a string representation of what our components should look like at that point in time, and react compares that with the current DOM and finds all the differences, based on those differences creates some DOM representations of just the relevant parts and updates the view.

Building DOM Representations

Since the HTML is defined in JavaScript it would get a bit hard to understand for larger pages with a lot of nesting, there would be curly braces everywhere, so for that reason JSX syntax is used to define the elements. This is very similar to other templating engines and uses ordinary HTML-type syntax.