ajax.js

function ajaxRequest(url, callback) {
  var req = new XMLHttpRequest();

  req.open('GET', url, true);
  req.onreadystatechange = function() {
    if (req.readyState == 4) {
      if (req.status == 200) {
        callback(req.responseText);
      }
    }
  };

  req.send(null);
}

assert.js

function assert(message, expr) {
  if(!expr) {
    throw new Error(message);
  }
  assert.count++;

  return true;
}

assert.count = 0;

core.js

var kamisori = {};

kamisori = {
  format: function(s) {
    var text = '';

    if (s.match(/=== ((\w|\W)+) ===/)) {
      text += '<h1>' + RegExp.$1 + '</h1>';
    } else if (s.match(/== ((\w|\W)+) ==/)) {
      text += '<h2>' + RegExp.$1 + '</h2>';
    } else if (s.match(/= ((\w|\W)+) =/)) {
      text += '<h3>' + RegExp.$1 + '</h3>';
    } else if (s.match(/\* ((\w|\W)+)/)) {
      text += '<ul>';
      text += '<li>' + RegExp.$1 + '</li>';
      text += '</ul>';
    } else {
      text += s;
    }

    return text;
  },

  output: function(path) {
    ajaxRequest(path, function(input) {
      var text = '';
      
      input = input.split('\n');
      for (var i = 0; i < input.length; i++) {
        if (input[i] != '') {
          text += kamisori.format(input[i]);
        }
      }

      var output = document.getElementById('out');
      output.innerHTML = text;
    });
  }
};

kamisori.output("in.txt");

core_test.js

try {
  assert('=== a === surrounded by h1', kamisori.format('=== a ===') == '<h1>a</h1>');
  assert('=== あ === surrounded by h1', kamisori.format('=== あ ===') == '<h1>あ</h1>');
  assert('=== a a === surrounded by h1', kamisori.format('=== a a ===') == '<h1>a a</h1>');
  assert('=== あ あ === surrounded by h1', kamisori.format('=== あ あ ===') == '<h1>あ あ</h1>')

  assert('== a == surrounded by h2', kamisori.format('== a ==') == '<h2>a</h2>');
  assert('== あ == surrounded by h2', kamisori.format('== あ ==') == '<h2>あ</h2>');
  assert('== a a == surrounded by h2', kamisori.format('== a a ==') == '<h2>a a</h2>');
  assert('== あ あ == surrounded by h2', kamisori.format('== あ あ ==') == '<h2>あ あ</h2>')

  assert('= a = surrounded by h3', kamisori.format('= a =') == '<h3>a</h3>');
  assert('= あ = surrounded by h3', kamisori.format('= あ =') == '<h3>あ</h3>');
  assert('= a a = surrounded by h3', kamisori.format('= a a =') == '<h3>a a</h3>');
  assert('= あ あ = surrounded by h3', kamisori.format('= あ あ =') == '<h3>あ あ</h3>')

  assert('* a surrounded by ul and li', kamisori.format('* a') == '<ul><li>a</li></ul>');
  assert('* あ surrounded by ul and li', kamisori.format('* あ') == '<ul><li>あ</li></ul>');
  assert('* a a surrounded by ul and li', kamisori.format('* a a') == '<ul><li>a a</li></ul>');
  assert('* あ あ surrounded by ul and li', kamisori.format('* あ あ') == '<ul><li>あ あ</li></ul>');

  console.log(assert.count + ' tests OK');
} catch(e) {
  console.log('Test failed: ' + e.message);
}