Javascript实现css/js/html懒加载

纯js实现css/js/hmtl懒加载，现在很多前端框架都有封装好的懒加载方式，那么原生该如何实现呢？话不多说　上代码～

'use strict';
var LazyLoader = (function() {
     

  function LazyLoader() {
     
    this._loaded = {
     };
    this._isLoading = {
     };
  }

  LazyLoader.prototype = {
     

    _js: function(file, callback) {
     
      var script = document.createElement('script');
      script.src = file;
      script.async = false;
      script.addEventListener('load', callback);     document.head.appendChild(script);
      this._isLoading[file] = script;
    },

    _css: function(file, callback) {
     
      var style = document.createElement('link');
      style.type = 'text/css';
      style.rel = 'stylesheet';
      style.href = file;
      document.head.appendChild(style);
      callback();
    },

    _html: function(domNode, callback) {
     

      if (domNode.getAttribute('is')) {
     
        this.load(['/shared/js/html_imports.js'], function() {
     
          HtmlImports.populate(callback);
        }.bind(this));
        return;
      }

      for (var i = 0; i < domNode.childNodes.length; i++) {
     
        if (domNode.childNodes[i].nodeType == document.COMMENT_NODE) {
     
          domNode.innerHTML = domNode.childNodes[i].nodeValue;
          break;
        }
      }

      window.dispatchEvent(new CustomEvent('lazyload', {
     
        detail: domNode
      }));

      callback();
    },
    getJSON: function(file, mozSystem) {
     
      return new Promise(function(resolve, reject) {
     
        var xhr;
        if (mozSystem) {
     
          xhr = new XMLHttpRequest({
     mozSystem: true});
        } else {
     
          xhr = new XMLHttpRequest();
        }
        xhr.open('GET', file, true);
        xhr.responseType = 'json';

        xhr.onerror = function(error) {
     
          reject(error);
        };
        xhr.onload = function() {
     
          if (xhr.response !== null) {
     
            resolve(xhr.response);
          } else {
     
            reject(new Error('No valid JSON object was found (' + 
            xhr.status + ' ' + xhr.statusText + ')'));
          }
        };

        xhr.send();
      });
    },

    load: function(files, callback) {
     
      var deferred = {
     };
      deferred.promise = new Promise(resolve => {
     
        deferred.resolve = resolve;
      });

      if (!Array.isArray(files)) {
     
        files = [files];
      }

      var loadsRemaining = files.length, self = this;
      function perFileCallback(file) {
     
        if (self._isLoading[file]) {
     
          delete self._isLoading[file];
        }
        self._loaded[file] = true;

        if (--loadsRemaining === 0) {
     
          deferred.resolve();
          if (callback) {
     
            callback();
          }
        }
      }

      for (var i = 0; i < files.length; i++) {
     
        var file = files[i];

        if (this._loaded[file.id || file]) {
     
          perFileCallback(file);
        } else if (this._isLoading[file]) {
     
          this._isLoading[file].addEventListener(
            'load', perFileCallback.bind(null, file));
        } else {
     
          var method, idx;
          if (typeof file === 'string') {
     
            method = file.match(/\.([^.]+)$/)[1];
            idx = file;
          } else {
     
            method = 'html';
            idx = file.id;
          }

          this['_' + method](file, perFileCallback.bind(null, idx));
        }
      }

      return deferred.promise;
    }
  };

  return new LazyLoader();
}());