old_contents: /**
   * index.js
   */
  
  var $ = require('jquery');
  var Promise = require('es6-promise').Promise;
  import Ajax from './ajax.babel.js';
  
  $(() => {
    /* Variables */
    let $gitName       = $('#github_name');
    let $yoName        = $('#yo_name');
    let $gitNameStatus = $('.git_status');
  
    let appUrl = location.protocol + '//' + location.host;
    let gitCheckAjax = new Ajax(appUrl + '/check/git');
    let registerAjax = new Ajax(appUrl + '/register/user');
  
    /* Functions */
    // draw GitHub user name status
    let showStatus = (status, isValid) => {
      $gitNameStatus.text(status);
      $gitNameStatus.css('color', isValid ? 'lightgreen' : 'red');
    };
  
    // parse JSON response and judge it is success
    let checkJson = (json) => {
      return json.status === 'success';
    };
  
    /* Event Listeners */
    // watch and check GitHub user name
    $gitName.change(() => {
      console.log('Debug: text box changed');
      gitCheckAjax.request({ git_name: $gitName.val() })
        .then((result) => {
          return checkJson(result);
        })
        .then((result) => {
          showStatus(result ? 'user name is valid' : 'invalid user name', result);
        })
        .catch(error => console.log(error));
    });
  });
  
new_contents: /**
   * index.js
   */
  
  var $ = require('jquery');
  var Promise = require('es6-promise').Promise;
  import Ajax from './ajax.babel.js';
  
  $(() => {
    /* Variables */
    let $gitName       = $('#github_name');
    let $yoName        = $('#yo_name');
    let $gitNameStatus = $('.git_status');
  
    let appUrl = location.protocol + '//' + location.host;
    let gitCheckAjax = new Ajax(appUrl + '/check/git');
    let registerAjax = new Ajax(appUrl + '/register/user');
  
    /* Functions */
    // draw GitHub user name status
    let showStatus = (status, isValid) => {
      $gitNameStatus.text(status);
      $gitNameStatus.css('color', isValid ? 'lightgreen' : 'red');
    };
  
    let checkStatus = () => {
      $gitNameStatus.text('now checking your GitHub name...');
      $gitNameStatus.css('color', 'blue');
    };
  
    // parse JSON response and judge it is success
    let checkJson = (json) => {
      return json.status === 'success';
    };
  
    /* Event Listeners */
    // watch and check GitHub user name
    $gitName.change(() => {
      console.log('Debug: text box changed');
      checkStatus();
      gitCheckAjax.request({ git_name: $gitName.val() })
        .then((result) => {
          return checkJson(result);
        })
        .then((result) => {
          showStatus(result ? 'user name is valid' : 'invalid user name', result);
        })
        .catch(error => console.log(error));
    });
  });
  
subject: add checking sentence