[docs] Glossary

Pulled in https://gist.github.com/sebmarkbage/fcb1b6ab493b0c77d589
mostly verbatim

docs/_data/nav_docs.yml

@@ -77,6 +77,8 @@
     title: Special Non-DOM Attributes
   - id: reconciliation
     title: Reconciliation
+  - id: glossary
+    title: React (Virtual) DOM Terminology
 - title: Flux
   items:
   - id: flux-overview

docs/docs/ref-08-reconciliation.md

@@ -3,6 +3,7 @@ id: reconciliation
 title: Reconciliation
 permalink: reconciliation.html
 prev: special-non-dom-attributes.html
+next: glossary.html
 ---
 
 React's key design decision is to make the API seem like it re-renders the whole app on every update. This makes writing applications a lot easier but is also an incredible challenge to make it tractable. This article explains how with powerful heuristics we managed to turn a O(n<sup>3</sup>) problem into a O(n) one.

docs/docs/ref-09-glossary.md

@@ -0,0 +1,192 @@
+---
+id: glossary
+title: React (Virtual) DOM Terminology
+permalink: glossary.html
+prev: reconciliation.html
+---
+
+In React's terminology, there are five core types that are important to distinguish:
+
+- [ReactElement / ReactElement Factory](#react-elements)
+- [ReactNode](#react-nodes)
+- [ReactComponent / ReactComponent Class](#react-components)
+
+## React Elements
+
+The primary type in React is the `ReactElement`. It has four properties: `type`, `props`, `key` and `ref`. It has no methods and nothing on the prototype.
+
+You can create one of these object through `React.createElement`.
+
+```javascript
+var root = React.createElement('div');
+```
+
+To render a new tree into the DOM, you create `ReactElement`s and pass them to `React.render` a long with a regular DOM `Element` (`HTMLElement` or `SVGElement`). `ReactElement`s are not to be confused with DOM `Element`s. A `ReactElement` is a light, stateless, immutable, virtual representation of a DOM `Element`. It is a virtual DOM.
+
+```javascript
+React.render(root, document.body);
+```
+
+To add properties to a DOM element, pass a properties object as the second argument and children to the third argument.
+
+```javascript
+var child = React.createElement('li', null, 'Text Content');
+var root = React.createElement('ul', { className: 'my-list' }, child);
+React.render(root, document.body);
+```
+
+If you use React JSX, then these `ReactElement`s are created for you. So this is equivalent:
+
+```javascript
+var root = <ul className="my-list">
+             <li>Text Content</li>
+           </ul>;
+React.render(root, document.body);
+```
+
+__Factories__
+
+A `ReactElement`-factory is simply a function that generates a `ReactElement` with a particular `type` property. React has a built-in helper for you to create factories. It's effectively just:
+
+```javascript
+function createFactory(type){
+  return React.createElement.bind(null, type);
+}
+```
+
+It allows you to create a convenient short-hand instead of typing out `React.createElement('div')` all the time.
+
+```javascript
+var div = React.createFactory('div');
+var root = div({ className: 'my-div' });
+React.render(root, document.body);
+```
+
+React already have built-in factories for common HTML tags:
+
+```javascript
+var root = React.DOM.ul({ className: 'my-list' },
+             React.DOM.li(null, 'Text Content')
+           );
+```
+
+If you are using JSX you have no need for factories. JSX already provides a convenient short-hand for creating `ReactElement`s.
+
+
+## React Nodes
+
+A `ReactNode` can be either:
+- `ReactElement`
+- `string` (aka `ReactText`)
+- `number` (aka `ReactText`)
+- Array of `ReactNode`s (aka `ReactFragment`)
+
+These are used as properties of other `ReactElement`s to represent children. Effectively they create a tree of `ReactElement`s.
+
+
+## React Components
+
+You can use React using only `ReactElement`s but to really take advantage of React, you'll want to use `ReactComponent`s to create encapsulations with embedded state.
+
+A `ReactComponent` Class is simply just a JavaScript class (or "constructor function").
+
+```javascript
+var MyComponent = React.createClass({
+  render: function() {
+    ...
+  }
+});
+```
+
+When this constructor is invoked it is expected to return an object with at least a `render` method on it. This object is referred to as a `ReactComponent`.
+
+```javascript
+var component = new MyComponent(props); // never do this
+```
+
+Other than for testing, you would normally __never__ call this constructor yourself. React calls it for you.
+
+Instead, you pass the `ReactComponent` Class to `createElement` you get a `ReactElement`.
+
+```javascript
+var element = React.createElement(MyComponent);
+```
+
+OR using JSX:
+
+```javascript
+var element = <MyComponent />;
+```
+
+When this is passed to `React.render`, React will call the constructor for you and create a `ReactComponent`, which returned.
+
+```javascript
+var component = React.render(element, document.body);
+```
+
+If you keep calling `React.render` with the same type of `ReactElement` and the same container DOM `Element` it always returns the same instance. This instance is stateful.
+
+```javascript
+var componentA = React.render(<MyComponent />, document.body);
+var componentB = React.render(<MyComponent />, document.body);
+componentA === componentB; // true
+```
+
+This is why you shouldn't construct your own instance. Instead, `ReactElement` is a virtual `ReactComponent` before it gets constructed. An old and new `ReactElement` can be compared to see if a new `ReactComponent` instance is created or if the existing one is reused.
+
+The `render` method of a `ReactComponent` is expected to return another `ReactElement`. This allows these components to be composed. Ultimately the render resolves into `ReactElement` with a `string` tag which instantiates a DOM `Element` instance and inserts it into the document.
+
+
+## Formal Type Definitions
+
+__Entry Point__
+
+```
+React.render = (ReactElement, HTMLElement | SVGElement) => ReactComponent;
+```
+
+__Nodes and Elements__
+
+```
+type ReactNode = ReactElement | ReactFragment | ReactText;
+
+type ReactElement = ReactComponentElement | ReactDOMElement;
+
+type ReactDOMElement = {
+  type : string,
+  props : {
+    children : ReactNodeList,
+    className : string,
+    etc.
+  },
+  key : string | boolean | number | null,
+  ref : string | null
+};
+
+type ReactComponentElement<TProps> = {
+  type : ReactClass<TProps>,
+  props : TProps,
+  key : string | boolean | number | null,
+  ref : string | null
+};
+
+type ReactFragment = Array<ReactNode | ReactEmpty>;
+
+type ReactNodeList = ReactNode | ReactEmpty;
+
+type ReactText = string | number;
+
+type ReactEmpty = null | undefined | boolean;
+```
+
+__Classes and Components__
+
+```
+type ReactClass<TProps> = (TProps) => ReactComponent<TProps>;
+
+type ReactComponent<TProps> = {
+  props : TProps,
+  render : () => ReactElement
+};
+```
+