diff --git a/docs/demo/demo.css b/docs/demo/demo.css
index 92d2f4a504..398c663a51 100644
--- a/docs/demo/demo.css
+++ b/docs/demo/demo.css
@@ -62,7 +62,11 @@ header h1 {
 	flex-grow: 1;
 }
 
-#options.badParse {
+#main {
+	display: none;
+}
+
+.error {
 	border-color: red;
 	background-color: #FEE
 }
diff --git a/docs/demo/demo.js b/docs/demo/demo.js
index 3762cb3819..de6882a5be 100644
--- a/docs/demo/demo.js
+++ b/docs/demo/demo.js
@@ -7,12 +7,21 @@ if (!window.fetch) {
   window.fetch = unfetch;
 }
 
+onunhandledrejection = function (e) {
+  throw e.reason;
+};
+
+var $loadingElem = document.querySelector('#loading');
+var $mainElem = document.querySelector('#main');
 var $markdownElem = document.querySelector('#markdown');
 var $markedVerElem = document.querySelector('#markedVersion');
+var $commitVerElem = document.querySelector('#commitVersion');
 var $markedVer = document.querySelector('#markedCdn');
 var $optionsElem = document.querySelector('#options');
 var $outputTypeElem = document.querySelector('#outputType');
 var $inputTypeElem = document.querySelector('#inputType');
+var $responseTimeElem = document.querySelector('#responseTime');
+var $previewElem = document.querySelector('#preview');
 var $previewIframe = document.querySelector('#preview iframe');
 var $permalinkElem = document.querySelector('#permalink');
 var $clearElem = document.querySelector('#clear');
@@ -20,81 +29,197 @@ var $htmlElem = document.querySelector('#html');
 var $lexerElem = document.querySelector('#lexer');
 var $panes = document.querySelectorAll('.pane');
 var $inputPanes = document.querySelectorAll('.inputPane');
+var lastInput = '';
 var inputDirty = true;
 var $activeOutputElem = null;
 var search = searchToObject();
-
 var markedVersions = {
   master: 'https://cdn.jsdelivr.net/gh/markedjs/marked/lib/marked.js'
 };
 var markedVersionCache = {};
+var delayTime = 1;
+var checkChangeTimeout = null;
+var markedWorker;
 
-var iframeLoaded = false;
-$previewIframe.addEventListener('load', function () {
-  iframeLoaded = true;
-  inputDirty = true;
+$previewIframe.addEventListener('load', handleIframeLoad);
+
+$outputTypeElem.addEventListener('change', handleOutputChange, false);
+
+$inputTypeElem.addEventListener('change', handleInputChange, false);
+
+$markedVerElem.addEventListener('change', handleVersionChange, false);
+
+$markdownElem.addEventListener('change', handleInput, false);
+$markdownElem.addEventListener('keyup', handleInput, false);
+$markdownElem.addEventListener('keypress', handleInput, false);
+$markdownElem.addEventListener('keydown', handleInput, false);
+
+$optionsElem.addEventListener('change', handleInput, false);
+$optionsElem.addEventListener('keyup', handleInput, false);
+$optionsElem.addEventListener('keypress', handleInput, false);
+$optionsElem.addEventListener('keydown', handleInput, false);
+
+$commitVerElem.style.display = 'none';
+$commitVerElem.addEventListener('keypress', handleAddVersion, false);
+
+$clearElem.addEventListener('click', handleClearClick, false);
+
+Promise.all([
+  setInitialQuickref(),
+  setInitialOutputType(),
+  setInitialText(),
+  setInitialVersion()
+    .then(setInitialOptions)
+]).then(function () {
+  handleInputChange();
+  handleOutputChange();
+  checkForChanges();
+  setScrollPercent(0);
+  $loadingElem.style.display = 'none';
+  $mainElem.style.display = 'block';
 });
 
-if ('text' in search && search.text) {
-  $markdownElem.value = search.text;
-} else {
-  fetch('./initial.md')
+function setInitialText() {
+  if ('text' in search) {
+    $markdownElem.value = search.text;
+  } else {
+    return fetch('./initial.md')
+      .then(function (res) { return res.text(); })
+      .then(function (text) {
+        if ($markdownElem.value === '') {
+          $markdownElem.value = text;
+        }
+      });
+  }
+}
+
+function setInitialQuickref() {
+  return fetch('./quickref.md')
     .then(function (res) { return res.text(); })
     .then(function (text) {
-      if ($markdownElem.value === '') {
-        $markdownElem.value = text;
-        inputDirty = true;
-        setScrollPercent(0);
-      }
+      document.querySelector('#quickref').value = text;
     });
 }
 
-fetch('https://data.jsdelivr.com/v1/package/npm/marked')
-  .then(function (res) {
-    return res.json();
-  })
-  .then(function (json) {
-    for (var i = 0; i < json.versions.length; i++) {
-      var ver = json.versions[i];
-      markedVersions[ver] = 'https://cdn.jsdelivr.net/npm/marked@' + ver + '/lib/marked.js';
-      var opt = document.createElement('option');
-      opt.textContent = ver;
-      opt.value = ver;
-      $markedVerElem.appendChild(opt);
-    }
-  })
-  .then(function () {
-    if ('version' in search && search.version) {
-      $markedVerElem.value = search.version;
-    } else {
-      $markedVerElem.value = 'master';
-    }
-
-    updateVersion().then(function () {
-      if ('options' in search && search.options) {
-        $optionsElem.value = search.options;
-      } else {
-        $optionsElem.value = JSON.stringify(
-          marked.getDefaults(),
-          function (key, value) {
-            if (value && typeof value === 'object' && Object.getPrototypeOf(value) !== Object.prototype) {
-              return undefined;
+function setInitialVersion() {
+  return fetch('https://data.jsdelivr.com/v1/package/npm/marked')
+    .then(function (res) {
+      return res.json();
+    })
+    .then(function (json) {
+      for (var i = 0; i < json.versions.length; i++) {
+        var ver = json.versions[i];
+        markedVersions[ver] = 'https://cdn.jsdelivr.net/npm/marked@' + ver + '/lib/marked.js';
+        var opt = document.createElement('option');
+        opt.textContent = ver;
+        opt.value = ver;
+        $markedVerElem.appendChild(opt);
+      }
+    })
+    .then(function () {
+      if (search.version) {
+        if (!markedVersions[search.version]) {
+          var match = search.version.match(/^(\w+):(.+)$/);
+          if (match) {
+            switch (match[1]) {
+              case 'commit':
+                addCommitVersion(search.version, match[2].substring(0, 7), match[2]);
+                return search.version;
+              case 'pr':
+                return getPrCommit(match[2])
+                  .then(function (commit) {
+                    if (!commit) {
+                      return 'master';
+                    }
+                    addCommitVersion(search.version, 'PR #' + match[2], commit);
+                    return search.version;
+                  });
             }
-            return value;
-          }, ' ');
+          }
+        }
       }
-    });
-  });
 
-if (search.outputType) {
-  $outputTypeElem.value = search.outputType;
+      return 'master';
+    })
+    .then(function (version) {
+      $markedVerElem.value = version;
+    })
+    .then(updateVersion);
+}
+
+function setInitialOptions() {
+  if ('options' in search) {
+    $optionsElem.value = search.options;
+  } else {
+    setDefaultOptions();
+  }
+}
+
+function setInitialOutputType() {
+  if (search.outputType) {
+    $outputTypeElem.value = search.outputType;
+  }
+}
+
+function handleIframeLoad() {
+  lastInput = '';
+  inputDirty = true;
+}
+
+function handleInput() {
+  inputDirty = true;
+};
+
+function handleVersionChange() {
+  if ($markedVerElem.value === 'commit' || $markedVerElem.value === 'pr') {
+    $commitVerElem.style.display = '';
+  } else {
+    $commitVerElem.style.display = 'none';
+    updateVersion();
+  }
+}
+
+function handleClearClick() {
+  $markdownElem.value = '';
+  $markedVerElem.value = 'master';
+  $commitVerElem.style.display = 'none';
+  updateVersion().then(setDefaultOptions);
 }
 
-fetch('./quickref.md')
-  .then(function (res) { return res.text(); })
-  .then(function (text) {
-    document.querySelector('#quickref').value = text;
-  });
+function handleAddVersion(e) {
+  if (e.which === 13) {
+    switch ($markedVerElem.value) {
+      case 'commit':
+        var commit = $commitVerElem.value.toLowerCase();
+        if (!commit.match(/^[0-9a-f]{40}$/)) {
+          alert('That is not a valid commit');
+          return;
+        }
+        addCommitVersion('commit:' + commit, commit.substring(0, 7), commit);
+        $markedVerElem.value = 'commit:' + commit;
+        $commitVerElem.style.display = 'none';
+        $commitVerElem.value = '';
+        updateVersion();
+        break;
+      case 'pr':
+        $commitVerElem.disabled = true;
+        var pr = $commitVerElem.value.replace(/\D/g, '');
+        getPrCommit(pr)
+          .then(function (commit) {
+            $commitVerElem.disabled = false;
+            if (!commit) {
+              alert('That is not a valid PR');
+              return;
+            }
+            addCommitVersion('pr:' + pr, 'PR #' + pr, commit);
+            $markedVerElem.value = 'pr:' + pr;
+            $commitVerElem.style.display = 'none';
+            $commitVerElem.value = '';
+            updateVersion();
+          });
+    }
+  }
+}
 
 function handleInputChange() {
   handleChange($inputPanes, $inputTypeElem.value);
@@ -118,40 +243,51 @@ function handleChange(panes, visiblePane) {
   return active;
 };
 
-$outputTypeElem.addEventListener('change', handleOutputChange, false);
-handleOutputChange();
-$inputTypeElem.addEventListener('change', handleInputChange, false);
-handleInputChange();
-$markedVerElem.addEventListener('change', updateVersion, false);
-
-function handleInput() {
-  inputDirty = true;
-};
+function addCommitVersion(value, text, commit) {
+  if (markedVersions[value]) {
+    return;
+  }
+  markedVersions[value] = 'https://cdn.jsdelivr.net/gh/markedjs/marked@' + commit + '/lib/marked.js';
+  var opt = document.createElement('option');
+  opt.textContent = text;
+  opt.value = value;
+  $markedVerElem.insertBefore(opt, $markedVerElem.firstChild);
+}
 
-$markdownElem.addEventListener('change', handleInput, false);
-$markdownElem.addEventListener('keyup', handleInput, false);
-$markdownElem.addEventListener('keypress', handleInput, false);
-$markdownElem.addEventListener('keydown', handleInput, false);
+function getPrCommit(pr) {
+  return fetch('https://api.github.com/repos/markedjs/marked/pulls/' + pr + '/commits')
+    .then(function (res) {
+      return res.json();
+    })
+    .then(function (json) {
+      return json[json.length - 1].sha;
+    }).catch(function () {
+      // return undefined
+    });
+}
 
-$optionsElem.addEventListener('change', handleInput, false);
-$optionsElem.addEventListener('keyup', handleInput, false);
-$optionsElem.addEventListener('keypress', handleInput, false);
-$optionsElem.addEventListener('keydown', handleInput, false);
+function setDefaultOptions() {
+  if (window.Worker) {
+    messageWorker({
+      task: 'defaults',
+      version: markedVersions[$markedVerElem.value]
+    });
+  } else {
+    var defaults = marked.getDefaults();
+    setOptions(defaults);
+  }
+}
 
-$clearElem.addEventListener('click', function () {
-  $markdownElem.value = '';
-  $markedVerElem.value = 'master';
-  updateVersion().then(function () {
-    $optionsElem.value = JSON.stringify(
-      marked.getDefaults(),
-      function (key, value) {
-        if (value && typeof value === 'object' && Object.getPrototypeOf(value) !== Object.prototype) {
-          return undefined;
-        }
-        return value;
-      }, ' ');
-  });
-}, false);
+function setOptions(opts) {
+  $optionsElem.value = JSON.stringify(
+    opts,
+    function (key, value) {
+      if (value && typeof value === 'object' && Object.getPrototypeOf(value) !== Object.prototype) {
+        return undefined;
+      }
+      return value;
+    }, ' ');
+}
 
 function searchToObject() {
   // modified from https://stackoverflow.com/a/7090123/806777
@@ -187,6 +323,7 @@ function getScrollSize() {
 
   return e.scrollHeight - e.clientHeight;
 };
+
 function getScrollPercent() {
   var size = getScrollSize();
 
@@ -196,6 +333,7 @@ function getScrollPercent() {
 
   return $activeOutputElem.scrollTop / size;
 };
+
 function setScrollPercent(percent) {
   $activeOutputElem.scrollTop = percent * getScrollSize();
 };
@@ -213,8 +351,12 @@ function updateLink() {
 }
 
 function updateVersion() {
+  if (window.Worker) {
+    handleInput();
+    return Promise.resolve();
+  }
   var promise;
-  if ($markedVerElem.value in markedVersionCache) {
+  if (markedVersionCache[$markedVerElem.value]) {
     promise = Promise.resolve(markedVersionCache[$markedVerElem.value]);
   } else {
     promise = fetch(markedVersions[$markedVerElem.value])
@@ -233,58 +375,145 @@ function updateVersion() {
   }).then(handleInput);
 }
 
-var delayTime = 1;
-var options = {};
 function checkForChanges() {
-  if (inputDirty && typeof marked !== 'undefined') {
+  if (inputDirty && $markedVerElem.value !== 'commit' && $markedVerElem.value !== 'pr' && (typeof marked !== 'undefined' || window.Worker)) {
     inputDirty = false;
 
     updateLink();
 
-    var startTime = new Date();
-
-    var scrollPercent = getScrollPercent();
-
+    var options = {};
+    var optionsString = $optionsElem.value || '{}';
     try {
-      var optionsString = $optionsElem.value || '{}';
       var newOptions = JSON.parse(optionsString);
       options = newOptions;
-      $optionsElem.classList.remove('badParse');
+      $optionsElem.classList.remove('error');
     } catch (err) {
-      $optionsElem.classList.add('badParse');
+      $optionsElem.classList.add('error');
     }
 
-    var lexed = marked.lexer($markdownElem.value, options);
-
-    var lexedList = [];
-
-    for (var i = 0; i < lexed.length; i++) {
-      var lexedLine = [];
-      for (var j in lexed[i]) {
-        lexedLine.push(j + ':' + jsonString(lexed[i][j]));
+    var version = markedVersions[$markedVerElem.value];
+    var markdown = $markdownElem.value;
+    var hash = version + markdown + optionsString;
+    if (lastInput !== hash) {
+      lastInput = hash;
+      if (window.Worker) {
+        delayTime = 100;
+        messageWorker({
+          task: 'parse',
+          version: version,
+          markdown: markdown,
+          options: options
+        });
+      } else {
+        var startTime = new Date();
+        var lexed = marked.lexer(markdown, options);
+        var lexedList = [];
+        for (var i = 0; i < lexed.length; i++) {
+          var lexedLine = [];
+          for (var j in lexed[i]) {
+            lexedLine.push(j + ':' + jsonString(lexed[i][j]));
+          }
+          lexedList.push('{' + lexedLine.join(', ') + '}');
+        }
+        var parsed = marked.parser(lexed, options);
+        var scrollPercent = getScrollPercent();
+        setParsed(parsed, lexedList.join('\n'));
+        setScrollPercent(scrollPercent);
+        var endTime = new Date();
+        delayTime = endTime - startTime;
+        setResponseTime(delayTime);
+        if (delayTime < 50) {
+          delayTime = 50;
+        } else if (delayTime > 500) {
+          delayTime = 1000;
+        }
       }
-      lexedList.push('{' + lexedLine.join(', ') + '}');
     }
+  }
+  checkChangeTimeout = window.setTimeout(checkForChanges, delayTime);
+};
 
-    var parsed = marked.parser(lexed, options);
-
-    if (iframeLoaded) {
-      $previewIframe.contentDocument.body.innerHTML = (parsed);
-    }
-    $htmlElem.value = (parsed);
-    $lexerElem.value = (lexedList.join('\n'));
+function setResponseTime(ms) {
+  var amount = ms;
+  var suffix = 'ms';
+  if (ms > 1000 * 60 * 60) {
+    amount = 'Too Long';
+    suffix = '';
+  } else if (ms > 1000 * 60) {
+    amount = '>' + Math.floor(ms / (1000 * 60));
+    suffix = 'm';
+  } else if (ms > 1000) {
+    amount = '>' + Math.floor(ms / 1000);
+    suffix = 's';
+  }
+  $responseTimeElem.textContent = amount + suffix;
+}
 
-    setScrollPercent(scrollPercent);
+function setParsed(parsed, lexed) {
+  try {
+    $previewIframe.contentDocument.body.innerHTML = parsed;
+  } catch (ex) {}
+  $htmlElem.value = parsed;
+  $lexerElem.value = lexed;
+}
 
-    var endTime = new Date();
-    delayTime = endTime - startTime;
-    if (delayTime < 50) {
-      delayTime = 50;
-    } else if (delayTime > 500) {
-      delayTime = 1000;
+function messageWorker(message) {
+  if (!markedWorker || markedWorker.working) {
+    if (markedWorker) {
+      clearTimeout(markedWorker.timeout);
+      markedWorker.terminate();
     }
+    markedWorker = new Worker('worker.js');
+    markedWorker.onmessage = function (e) {
+      clearTimeout(markedWorker.timeout);
+      markedWorker.working = false;
+      switch (e.data.task) {
+        case 'defaults':
+          setOptions(e.data.defaults);
+          break;
+        case 'parse':
+          $previewElem.classList.remove('error');
+          $htmlElem.classList.remove('error');
+          $lexerElem.classList.remove('error');
+          var scrollPercent = getScrollPercent();
+          setParsed(e.data.parsed, e.data.lexed);
+          setScrollPercent(scrollPercent);
+          setResponseTime(e.data.time);
+          break;
+      }
+      clearTimeout(checkChangeTimeout);
+      delayTime = 10;
+      checkForChanges();
+    };
+    markedWorker.onerror = markedWorker.onmessageerror = function (err) {
+      clearTimeout(markedWorker.timeout);
+      var error = 'There was an error in the Worker';
+      if (err) {
+        if (err.message) {
+          error = err.message;
+        } else {
+          error = err;
+        }
+      }
+      error = error.replace(/^Uncaught Error: /, '');
+      $previewElem.classList.add('error');
+      $htmlElem.classList.add('error');
+      $lexerElem.classList.add('error');
+      setParsed(error, error);
+      setScrollPercent(0);
+    };
   }
-  window.setTimeout(checkForChanges, delayTime);
-};
-checkForChanges();
-setScrollPercent(0);
+  if (message.task !== 'defaults') {
+    markedWorker.working = true;
+    workerTimeout(0);
+  }
+  markedWorker.postMessage(message);
+}
+
+function workerTimeout(seconds) {
+  markedWorker.timeout = setTimeout(function () {
+    seconds++;
+    markedWorker.onerror('Marked has taken longer than ' + seconds + ' second' + (seconds > 1 ? 's' : '') + ' to respond...');
+    workerTimeout(seconds);
+  }, 1000);
+}
diff --git a/docs/demo/index.html b/docs/demo/index.html
index 115b26696e..96a8ec14b1 100644
--- a/docs/demo/index.html
+++ b/docs/demo/index.html
@@ -18,53 +18,55 @@
       <h1>Marked Demo</h1>
     </header>
 
-    <div class="containers">
-      <div class="container">
-        <div class="label">
-          <span>Input</span> ·
-          <a id="permalink">Permalink</a> ·
-          <span>Version: </span>
-          <select id="markedVersion">
-            <option value="master">master</option>
-          </select> ·
-          <button id="clear">Clear</button>
-          <select id="inputType">
-            <option value="markdown">Markdown</option>
-            <option value="options">Options</option>
-          </select>
-          <select id="markedVersion">
-            <option value="https://cdn.jsdelivr.net/npm/marked@master/marked.min.js">Marked@master</option>
-            <option value="https://cdn.jsdelivr.net/npm/marked@0.6.0/marked.min.js">Marked@0.6.0</option>
-            <option value="https://cdn.jsdelivr.net/npm/marked@0.5.2/marked.min.js">Marked@0.5.2</option>
-            <option value="https://cdn.jsdelivr.net/npm/marked@0.5.1/marked.min.js">Marked@0.5.1</option>
-          </select>
+    <div id="loading">Loading...</div>
+    <div id="main">
+      <div class="containers">
+        <div class="container">
+          <div class="label">
+            <span>Input</span> ·
+            <a id="permalink">Permalink</a> ·
+            <span>Version: </span>
+            <select id="markedVersion">
+              <option value="pr">Add PR:</option>
+              <option value="commit">Add Commit:</option>
+              <option value="master" selected>master</option>
+            </select>
+            <input type="text" id="commitVersion" title="Press ENTER to add commit" />·
+            <button id="clear">Clear</button>
+            <select id="inputType">
+              <option value="markdown">Markdown</option>
+              <option value="options">Options</option>
+            </select>
+          </div>
+          <textarea id="markdown" class="inputPane"></textarea>
+          <textarea id="options" class="inputPane" placeholder="Options (as JSON)"></textarea>
         </div>
-        <textarea id="markdown" class="inputPane"></textarea>
-        <textarea id="options" class="inputPane" placeholder="Options (as JSON)"></textarea>
-      </div>
 
-      <div class="container">
-        <div class="label">
-          <select id="outputType">
-            <option value="preview">Preview</option>
-            <option value="html">HTML Source</option>
-            <option value="lexer">Lexer Data</option>
-            <option value="quickref">Quick Reference</option>
-          </select>
-        </div>
+        <div class="container">
+          <div class="label">
+            <select id="outputType">
+              <option value="preview">Preview</option>
+              <option value="html">HTML Source</option>
+              <option value="lexer">Lexer Data</option>
+              <option value="quickref">Quick Reference</option>
+            </select> ·
+            Response Time:
+            <span id="responseTime"></span>
+          </div>
 
-        <div id="preview" class="pane">
-          <noscript>
-            <h2>You'll need to enable Javascript to use this tool.</h2>
-          </noscript>
-          <iframe src="./preview.html" frameborder="0" sandbox="allow-same-origin"></iframe>
-        </div>
+          <div id="preview" class="pane">
+            <noscript>
+              <h2>You'll need to enable Javascript to use this tool.</h2>
+            </noscript>
+            <iframe src="./preview.html" frameborder="0" sandbox="allow-same-origin allow-top-navigation-by-user-activation"></iframe>
+          </div>
 
-        <textarea id="html" class="pane" readonly="readonly"></textarea>
+          <textarea id="html" class="pane" readonly="readonly"></textarea>
 
-        <textarea id="lexer" class="pane" readonly="readonly"></textarea>
+          <textarea id="lexer" class="pane" readonly="readonly"></textarea>
 
-        <textarea id="quickref" class="pane" readonly="readonly"></textarea>
+          <textarea id="quickref" class="pane" readonly="readonly"></textarea>
+        </div>
       </div>
     </div>
     <script id="markedCdn"></script>
diff --git a/docs/demo/initial.md b/docs/demo/initial.md
index 2465aef041..d2b7d77c10 100644
--- a/docs/demo/initial.md
+++ b/docs/demo/initial.md
@@ -30,7 +30,7 @@ It's easy.  It's not overly bloated, unlike HTML.  Also, as the creator of [mark
 > or formatting instructions.
 
 Ready to start writing?  Either start changing stuff on the left or
-[clear everything](?text=) with a simple click.
+[clear everything](/demo/?text=) with a simple click.
 
 [Marked]: https://github.com/markedjs/marked/
 [Markdown]: http://daringfireball.net/projects/markdown/
diff --git a/docs/demo/worker.js b/docs/demo/worker.js
new file mode 100644
index 0000000000..06b8befe78
--- /dev/null
+++ b/docs/demo/worker.js
@@ -0,0 +1,105 @@
+/* globals marked, unfetch, ES6Promise */
+if (!self.Promise) {
+  self.importScripts('https://cdn.jsdelivr.net/npm/es6-promise/dist/es6-promise.js');
+  self.Promise = ES6Promise;
+}
+if (!self.fetch) {
+  self.importScripts('https://cdn.jsdelivr.net/npm/unfetch/dist/unfetch.umd.js');
+  self.fetch = unfetch;
+}
+
+var versionCache = {};
+var currentVersion;
+
+onunhandledrejection = function (e) {
+  throw e.reason;
+};
+
+onmessage = function (e) {
+  if (e.data.version === currentVersion) {
+    parse(e);
+  } else {
+    loadVersion(e.data.version).then(function () {
+      parse(e);
+    });
+  }
+};
+
+function parse(e) {
+  switch (e.data.task) {
+    case 'defaults':
+
+      var defaults = {};
+      if (typeof marked.getDefaults === 'function') {
+        defaults = marked.getDefaults();
+        delete defaults.renderer;
+      } else if ('defaults' in marked) {
+        for (var prop in marked.defaults) {
+          if (prop !== 'renderer') {
+            defaults[prop] = marked.defaults[prop];
+          }
+        }
+      }
+      postMessage({
+        task: e.data.task,
+        defaults: defaults
+      });
+      break;
+    case 'parse':
+      var startTime = new Date();
+      var lexed = marked.lexer(e.data.markdown, e.data.options);
+      var lexedList = [];
+      for (var i = 0; i < lexed.length; i++) {
+        var lexedLine = [];
+        for (var j in lexed[i]) {
+          lexedLine.push(j + ':' + jsonString(lexed[i][j]));
+        }
+        lexedList.push('{' + lexedLine.join(', ') + '}');
+      }
+      var parsed = marked.parser(lexed, e.data.options);
+      var endTime = new Date();
+      // setTimeout(function () {
+      postMessage({
+        task: e.data.task,
+        lexed: lexedList.join('\n'),
+        parsed: parsed,
+        time: endTime - startTime
+      });
+      // }, 10000);
+      break;
+  }
+}
+
+function jsonString(input) {
+  var output = (input + '')
+    .replace(/\n/g, '\\n')
+    .replace(/\r/g, '\\r')
+    .replace(/\t/g, '\\t')
+    .replace(/\f/g, '\\f')
+    .replace(/[\\"']/g, '\\$&')
+    .replace(/\u0000/g, '\\0');
+  return '"' + output + '"';
+};
+
+function loadVersion(ver) {
+  var promise;
+  if (versionCache[ver]) {
+    promise = Promise.resolve(versionCache[ver]);
+  } else {
+    promise = fetch(ver)
+      .then(function (res) { return res.text(); })
+      .then(function (text) {
+        versionCache[ver] = text;
+        return text;
+      });
+  }
+  return promise.then(function (text) {
+    try {
+      // eslint-disable-next-line no-new-func
+      Function(text)();
+    } catch (err) {
+      throw new Error('Cannot load that version of marked');
+    }
+    currentVersion = ver;
+  });
+}