A Markdown parser in under 100 JS LoC, taken from http://pzxc.com/simple-javascript-markdown-parsing-function

markdown.parse.js

const parse = (s) => {
    
    let r = s
    let ii
    let pre1 = []
    let pre2 = []

    // detect newline format
    const newline = r.indexOf('\r\n') != -1 ? '\r\n' : r.indexOf('\n') != -1 ? '\n' : ''

    // store {{{ unformatted blocks }}} and <pre> pre-formatted blocks </pre>
    r = r.replace(/{{{([\s\S]*?)}}}/g, x => { 
        pre1.push(x.substring(3, x.length - 3))
        return '{{{}}}'
    })
    r = r.replace(new RegExp('<pre>([\\s\\S]*?)</pre>', 'gi'), x => { 
        pre2.push(x.substring(5, x.length - 6))
        return '<pre></pre>'
    })

    // h1 - h4 and hr
    r = r.replace(/^==== (.*)=*/gm, '<h4>$1</h4>')
    r = r.replace(/^=== (.*)=*/gm, '<h3>$1</h3>')
    r = r.replace(/^== (.*)=*/gm, '<h2>$1</h2>')
    r = r.replace(/^= (.*)=*/gm, '<h1>$1</h1>')
    r = r.replace(/^[-*][-*][-*]+/gm, '<hr>')

    // bold, italics, and code formatting
    r = r.replace(/\*\*(.*?)\*\*/g, '<strong>$1</strong>')
    r = r.replace(new RegExp('//(((?!https?://).)*?)//', 'g'), '<em>$1</em>')
    r = r.replace(/``(.*?)``/g, '<code>$1</code>')

    // unordered lists
    r = r.replace(/^\*\*\*\* (.*)/gm, '<ul><ul><ul><ul><li>$1</li></ul></ul></ul></ul>')
    r = r.replace(/^\*\*\* (.*)/gm, '<ul><ul><ul><li>$1</li></ul></ul></ul>')
    r = r.replace(/^\*\* (.*)/gm, '<ul><ul><li>$1</li></ul></ul>')
    r = r.replace(/^\* (.*)/gm, '<ul><li>$1</li></ul>')
    for (ii = 0; ii < 3; ii++)
    {
        r = r.replace(new RegExp('</ul>' + newline + '<ul>', 'g'), newline)
    }

    // ordered lists
    r = r.replace(/^#### (.*)/gm, '<ol><ol><ol><ol><li>$1</li></ol></ol></ol></ol>')
    r = r.replace(/^### (.*)/gm, '<ol><ol><ol><li>$1</li></ol></ol></ol>')
    r = r.replace(/^## (.*)/gm, '<ol><ol><li>$1</li></ol></ol>')
    r = r.replace(/^# (.*)/gm, '<ol><li>$1</li></ol>')
    for (ii = 0; ii < 3; ii++)
    {
        r = r.replace(new RegExp('</ol>' + newline + '<ol>', 'g'), newline)
    }

    // links
    r = r.replace(/\[\[(http:[^\]|]*?)\]\]/g, '<a target="_blank" href="$1">$1</a>')
    r = r.replace(/\[\[(http:[^|]*?)\|(.*?)\]\]/g, '<a target="_blank" href="$1">$2</a>')
    r = r.replace(/\[\[([^\]|]*?)\]\]/g, '<a href="$1">$1</a>')
    r = r.replace(/\[\[([^|]*?)\|(.*?)\]\]/g, '<a href="$1">$2</a>')

    // images
    r = r.replace(/{{([^\]|]*?)}}/g, '<img src="$1">')
    r = r.replace(/{{([^|]*?)\|(.*?)}}/g, '<img src="$1" alt="$2">')

    // video
    r = r.replace(/<<(.*?)>>/g, '<video src="$1" allowfullscreen="true"></video>')

    // hard linebreak if there are 2 or more spaces at the end of a line
    r = r.replace(new RegExp(' + ' + newline, 'g'), '<br>' + newline)

    // split on double-newlines, then add paragraph tags when the first tag isn't a block level element
    if (newline != '') 
    {
        for (let p = r.split(newline + newline), i = 0; i < p.length; i++) 
        {
            let blockLevel = false
            if (p[i].length >= 1 && p[i].charAt(0) == '<') 
            {
                // check if the first tag is a block-level element
                const firstSpace = p[i].indexOf(' '), firstCloseTag = p[i].indexOf('>')
                const endIndex = firstSpace > -1 && firstCloseTag > -1 ? Math.min(firstSpace, firstCloseTag) : firstSpace > -1 ? firstSpace : firstCloseTag
                const tag = p[i].substring(1, endIndex).toLowerCase()
                for (var j = 0; j < blockLevelElements.length; j++) 
                {
                    if (blockLevelElements[j] == tag) blockLevel = true
                }
            } else if (p[i].length >= 1 && p[i].charAt(0) == '|') {
                // format the paragraph as a table
                blockLevel = true
                p[i] = p[i].replace(/ \|= /g, '</th><th>').replace(/\|= /g, '<tr><th>').replace(/ \|=/g, '</th></tr>')
                p[i] = p[i].replace(/ \| /g, '</td><td>').replace(/\| /g, '<tr><td>').replace(/ \|/g, '</td></tr>')
                p[i] = '<table>' + p[i] + '</table>'
            } else if (p[i].length >= 2 && p[i].charAt(0) == '>' && p[i].charAt(1) == ' ') {
                // format the paragraph as a blockquote
                blockLevel = true
                p[i] = '<blockquote>' + p[i].replace(/^> /gm, '') + '</blockquote>'
            }
            if (!blockLevel) p[i] = '<p>' + p[i] + '</p>'
        }
    }
    
    // reassemble the paragraphs
    if (newline != '')
    {
        r = p.join(newline + newline) 
    }

    // restore the preformatted and unformatted blocks
    r = r.replace( new RegExp('<pre></pre>', 'g'),  match => '<pre>' + pre2.shift() + '</pre>' )
    r = r.replace(/{{{}}}/g, match => pre1.shift() )
    
    return r
}

mdown.js

    var mdown = function (s) {
        var r = s;

        // detect newline format
        var newline = r.indexOf('\r\n') != -1 ? '\r\n' : r.indexOf('\n') != -1 ? '\n' : ''

        // h1 - h2 and hr
        r = r.replace(/^== (.*)=*/gm, '<h2>$1</h2>');
        r = r.replace(/^= (.*)=*/gm, '<h1>$1</h1>');
        r = r.replace(/^[-*][-*][-*]+/gm, '<hr>');

        // bold, italics
        r = r.replace(/\*\*(.*?)\*\*/g, '<strong>$1</strong>');
        r = r.replace(new RegExp('//(((?!https?://).)*?)//', 'g'), '<em>$1</em>');

        // unordered lists
        r = r.replace(/^\* (.*)/gm, '<ul><li>$1</li></ul>');
        r = r.replace(new RegExp('</ul>' + newline + '<ul>', 'g'), newline);

        // links & images
        r = r.replace(/\[\[([^|]*?)\|(.*?)\]\]/g, '<a href="$1">$2</a>');
        r = r.replace(/{{([^\]|]*?)}}/g, '<img src="$1">');

        // split on double-newlines, then add paragraph tags when the first tag isn't a block level element
        if(newline != '') {
            for(var p = r.split(newline + newline), i = 0; i < p.length; i++) {
                p[i] = '<p>' + p[i] + '</p>';
            }

            r = p.join(newline + newline); // reassemble the paragraphs
        }
        return r;
    }