Javascript to convert Markdown/Textile to HTML (and, ideally, back to Markdown/Textile)

For Markdown -> HTML, there is Showdown

StackOverflow itself uses Markdown language for questions and answers ; did you try to take a look at how it works ?

Well, it seems it is using PageDown which is available under the MIT License

The question Is there any good Markdown Javascript library or control? and its answers might help, too :-)

A full editor is, of course, not exactly what you asked for ; but they must use some kind of function to transform the Markdown code to HTML ; and, depending on the license of these editors, you might be able to re-use that function...

Actually, if you take a close look at Showdown, in its code source (file showdown.js), you'll find this portion of comment :

//
// Showdown usage:
//
//   var text = "Markdown *rocks*.";
//
//   var converter = new Showdown.converter();
//   var html = converter.makeHtml(text);
//
//   alert(html);
//
// Note: move the sample code to the bottom of this
// file before uncommenting it.
//

It's not jQuery syntax, but should be quite easy to integrate in your application ;-)

About Textile, it seems to be a bit harder to find anything useful :-(

In the other side, HTML -> Markdown, I guess things might be a bit harder...

What I would do is store both Markdown and HTML in my application data store (database ? ), and use one for editing, and the other for rendering... Would take more space, but it seems less risky than "decrypting" HTML...

I thought it would be worthwhile to make a list here of the JavaScript solutions out there and their minified (uncompressed) size and strengths/weaknesses. Compressed size for minified code will be around 50% of uncompressed size. What it comes down to:

• Use markdown-it (104KB) if you need comprehensive support and will have user edited or arbitrary documents, but are not overly concerned with size/bandwidth.
• Use my own drawdown (1.3KB) if you need reasonably high quality, and table support, but want feather weight and do not need any features other than conversion, or to have every single edge case addressed.
• Use one of the others if you need unique capabilities like security or expandability.

All of these use the MIT license, most are on npm.

• markdown-it: 104KB. Powers StackExchange since the CommonMark migration. Follows the CommonMark spec and is now more or less the gold standard; supports syntax extensions; produces secure output by default. Fast; as robust as showdown, but very large. Has a ton of features (e.g. synced scrolling). Is also the basis for http://dillinger.io/.

• showdown: 28KB. Has comprehensive CommonMark support and was previously the gold standard; is significantly smaller than Markdown-It but slower. it is the basis for pagedown.

• pagedown: 8KB. Powered StackExchange before the CommonMark migration. It is very robust but missing tables, definition lists, footnotes, etc. In addition to the 8KB converter script, it also offers editor and sanitizer scripts.

• drawdown: 1.3KB. Full disclosure, I wrote it. Broader feature scope than any other lightweight converter; handles most but not all of the CommonMark spec. Not recommended for user editing but very useful for presenting information in web apps. No inline HTML.

• marked: 19KB. Comprehensive; tested against unit test suite; supports custom lexer rules.

• micromarkdown: 5KB. Supports a lot of features, but is missing some common ones like unordered lists using * and some common ones that aren't strictly part of the spec like fenced code blocks. Many bugs, throws exceptions on most longer documents. I consider it experimental.

• nano-markdown: 1.9KB. Feature scope limited to things used by most documents; more robust than micromarkdown but not perfect; uses its own very basic unit test. Reasonably robust but breaks on many edge cases.

• mmd.js: 800 bytes. The result of an effort to make the smallest possible parser that is still functional. Supports a small subset; document needs to be tailored for it.

• markdown-js: 54KB (not available for download minified; would probably minify to ~20KB). Looks pretty comprehensive and includes tests, but I'm not very familiar with it.

• meltdown: 41KB (not available for download minified; would probably minify to ~15KB). jQuery plugin; Markdown Extra (tables, definition lists, footnotes).

• unified.js: varies, 5-100KB. A plugin-based system for converting between html, markdown, and prose. Depending on what plugins you need (spell-checking, syntax-highlighting, input sanitizing) the file size will vary. Probably used more server-side than client-side.

Textile

You can find a seemingly very fine Javascript implementation of Textile here, and another one there (maybe not so good, but has a nice convert-as-you-type example page).

Note: there is a bug in the first implementation I made a link to : horizontal bars are not rendered correctly. To fix it, you can add the following code in the file.

for(i=0;i<lines.length;i++) {
    // Add code :Start
    if (lines[i].match(/\s*-{4,}\s*/)){
      html+="<hr/>\n";
      continue;
    }
    // Add code :End
    if (lines[i].indexOf("[") == 0) {continue;}
    //...