text: 'use strict';
  
  function readContent(filename, urlSuffix, isAddHistory) {
    if (isAddHistory == undefined) {
      isAddHistory = true;
    }
    var httpReq = new XMLHttpRequest();
    httpReq.open('GET', filename, true);
    httpReq.onreadystatechange = function() {
      if (httpReq.readyState == 4) {
        document.getElementById('main-area').innerHTML = httpReq.responseText;
        if (isAddHistory) {
          var state = {
            filename: filename,
            urlSuffix: urlSuffix
          };
          history.pushState(state, null, urlSuffix);
        }
      }
    }
    httpReq.send(null);
  }
  
  window.addEventListener('popstate', function(e) {
    readContent(history.state.filename, history.state.urlSuffix, false);
  }, false);
  