Reorganize and expand README (#673)

* Reorganize and expand README

* Document options & add example for isProbablyReaderable()

Author: danburzo

README.md

@@ -2,86 +2,103 @@
 
 A standalone version of the readability library used for Firefox Reader View.
 
-## Usage on the web
+## Basic usage
 
-To parse a document, you must create a new `Readability` object from a DOM document object, and then call `parse()`. Here's an example:
+To parse a document, you must create a new `Readability` object from a DOM document object, and then call the [`parse()`](#parse) method. Here's an example:
 
 ```javascript
 var article = new Readability(document).parse();
 ```
 
-This `article` object will contain the following properties:
+If you use Readability in a web browser, you will likely be able to use a `document` reference from elsewhere (e.g. fetched via XMLHttpRequest, in a same-origin `<iframe>` you have access to, etc.). In Node.js, you can [use an external DOM library](#nodejs-usage).
 
-* `title`: article title
-* `content`: HTML string of processed article content
-* `textContent`: text content of the article (all HTML removed)
-* `length`: length of an article, in characters
-* `excerpt`: article description, or short excerpt from the content
-* `byline`: author metadata
-* `dir`: content direction
+## API Reference
 
-If you're using Readability on the web, you will likely be able to use a `document` reference
-from elsewhere (e.g. fetched via XMLHttpRequest, in a same-origin `<iframe>` you have access to, etc.).
+### `new Readability(document, options)`
 
-### Optional
+The `options` object accepts a number of properties, all optional:
 
-Readability's `parse()` works by modifying the DOM. This removes some elements in the web page.
-You could avoid this by passing the clone of the `document` object while creating a `Readability` object.
+* `debug` (boolean, default `false`): whether to enable logging.
+* `maxElemsToParse` (number, default `0` i.e. no limit): the maximum number of elements to parse.
+* `nbTopCandidates` (number, default `5`): the number of top candidates to consider when analysing how tight the competition is among candidates.
+* `charThreshold` (number, default `500`): the number of characters an article must have in order to return a result.
+* `classesToPreserve` (array): a set of classes to preserve on HTML elements when the `keepClasses` options is set to `false`.
+* `keepClasses` (boolean, default `false`): whether to preserve all classes on HTML elements. When set to `false` only classes specified in the `classesToPreserve` array are kept.
+* `disableJSONLD` (boolean, default `false`): when extracting page metadata, Readability gives precendence to Schema.org fields specified in the JSON-LD format. Set this option to false to skip JSON-LD parsing.
+* `serializer` (function, default `el => el.innerHTML`) controls how the the `content` property returned by the `parse()` method is produced from the root DOM element. It may be useful to specify the `serializer` as the identity function (`el => el`) to obtain a DOM element instead of a string for `content` if you plan to process it further.
 
-```
+### `parse()`
+
+Returns an object containing the following properties:
+
+* `title`: article title;
+* `content`: HTML string of processed article content;
+* `textContent`: text content of the article, with all the HTML tags removed;
+* `length`: length of an article, in characters;
+* `excerpt`: article description, or short excerpt from the content;
+* `byline`: author metadata;
+* `dir`: content direction;
+* `siteName`: name of the site.
+
+The `parse()` method works by modifying the DOM. This removes some elements in the web page, which may be undesirable. You can avoid this by passing the clone of the `document` object to the `Readability` constructor:
+
+```js
 var documentClone = document.cloneNode(true); 
 var article = new Readability(documentClone).parse();
 ```
 
-## Usage from Node.js
+### `isProbablyReaderable(document, options)`
+
+A quick-and-dirty way of figuring out if it's plausible that the contents of a given document are suitable for processing with Readability. It is likely to produce both false positives and false negatives. The reason it exists is to avoid bogging down a time-sensitive process (like loading and showing the user a webpage) with the complex logic in the core of Readability. Improvements to its logic (while not deteriorating its performance) are very welcome.
+
+The `options` object accepts a number of properties, all optional:
+
+* `minContentLength` (number, default `140`): the minimum node content length used to decide if the document is readerable;
+* `minScore` (number, default `20`): the minumum cumulated 'score' used to determine if the document is readerable;
+* `visibilityChecker` (function, default `isNodeVisible`): the function used to determine if a node is visible;
+
+The function returns a boolean corresponding to whether or not we suspect `Readability.parse()` will suceeed at returning an article object. Here's an example:
+
+```js
+/* 
+    Only instantiate Readability  if we suspect 
+    the `parse()` method will produce a meaningful result.
+*/
+if (isProbablyReaderable(document)) {
+    let article = new Readability(document).parse();
+}
+```
+
+## Node.js usage
 
 Readability is available on npm:
 
 ```bash
 npm install @mozilla/readability
 ```
 
-In Node.js, you won't generally have a DOM document object. To obtain one, you can use external
-libraries like [jsdom](https://github.com/jsdom/jsdom). While this repository contains a parser of
-its own (`JSDOMParser`), that is restricted to reading XML-compatible markup and therefore we do
-not recommend it for general use.
-
-If you're using `jsdom` to create a DOM object, you should ensure that the page doesn't run (page)
-scripts (avoid fetching remote resources etc.) as well as passing it the page's URI as the `url`
-property of the `options` object you pass the `JSDOM` constructor.
-
-### Example:
+Since Node.js does not come with its own DOM implementation, we rely on external libraries like [jsdom](https://github.com/jsdom/jsdom). Here's an example using `jsdom` to obtain a DOM document object:
 
 ```js
 var { Readability } = require('@mozilla/readability');
-var JSDOM = require('jsdom').JSDOM;
-var doc = new JSDOM("<body>Here's a bunch of text</body>", {
+var { JSDOM } = require('jsdom');
+var doc = new JSDOM("<body>Look at this cat: <img src='./cat.jpg'></body>", {
   url: "https://www.example.com/the-page-i-got-the-source-from"
 });
 let reader = new Readability(doc.window.document);
 let article = reader.parse();
 ```
 
-## What's Readability-readerable?
+Remember to pass the page's URI as the `url` option in the `JSDOM` constructor (as shown in the example above), so that Readability can convert relative URLs for images, hyperlinks etc. to their absolute counterparts.
 
-It's a quick-and-dirty way of figuring out if it's plausible that the contents of a given
-document are suitable for processing with Readability. It is likely to produce both false
-positives and false negatives. The reason it exists is to avoid bogging down a time-sensitive
-process (like loading and showing the user a webpage) with the complex logic in the core of
-Readability. Improvements to its logic (while not deteriorating its performance) are very
-welcome.
+`jsdom` has the ability to run the scripts included in the HTML and fetch remote resources. For security reasons these are [disabled by default](https://github.com/jsdom/jsdom#executing-scripts), and we **strongly** recommend you keep them that way.
 
 ## Security
 
-If you're going to use Readability with untrusted input (whether in HTML or DOM form), we
-**strongly** recommend you use a sanitizer library like
-[DOMPurify](https://github.com/cure53/DOMPurify) to avoid script injection when you use
-the output of Readability. We would also recommend using
-[CSP](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) to add further defense-in-depth
+If you're going to use Readability with untrusted input (whether in HTML or DOM form), we **strongly** recommend you use a sanitizer library like [DOMPurify](https://github.com/cure53/DOMPurify) to avoid script injection when you use
+the output of Readability. We would also recommend using [CSP](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) to add further defense-in-depth
 restrictions to what you allow the resulting content to do. The Firefox integration of
-reader mode uses both of these techniques itself. Sanitizing unsafe content out of the input
-is explicitly not something we aim to do as part of Readability itself - there are other
-good sanitizer libraries out there, use them!
+reader mode uses both of these techniques itself. Sanitizing unsafe content out of the input is explicitly not something we aim to do as part of Readability itself - there are other good sanitizer libraries out there, use them!
 
 ## Contributing