也许是这样的：

<script>
if(!window.jQuery)
{
   var script = document.createElement('script');
   script.type = "text/javascript";
   script.src = "path/to/jQuery";
   document.getElementsByTagName('head')[0].appendChild(script);
}
</script>

避免使用“if (!jQuery)”，因为 IE 将返回错误：jQuery is 'undefined'

而是使用： if (typeof jQuery == 'undefined')

<script type="text/javascript">
if (typeof jQuery == 'undefined') {
    var script = document.createElement('script');
    script.type = "text/javascript";
    script.src = "http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js";
    document.getElementsByTagName('head')[0].appendChild(script);
}
</script>

您还需要在将 JQuery 附加到标头后检查它是否已加载。否则你将不得不等待 window.onload 事件，如果页面有图像，它会更慢。这是一个示例脚本，用于检查 JQuery 文件是否已加载，因为您无法方便地使用 $(document).ready(function...

http://neighborhood.org/core/sample/jquery/append-to-head.htm

方法一：

if (window.jQuery) {  
    // jQuery is loaded  
} else {
    // jQuery is not loaded
}

方法二：

if (typeof jQuery == 'undefined') {  
    // jQuery is not loaded
} else {
    // jQuery is loaded
}

如果未加载 jquery.js 文件，我们可以像这样强制加载它：

if (!window.jQuery) {
  var jq = document.createElement('script'); jq.type = 'text/javascript';
  // Path to jquery.js file, eg. Google hosted version
  jq.src = '/path-to-your/jquery.min.js';
  document.getElementsByTagName('head')[0].appendChild(jq);
}

试试这个 ：

<script>
  window.jQuery || document.write('<script src="js/jquery.min.js"><\/script>')
</script>

这将检查 jQuery 是否可用，如果不可用，它将从指定的路径动态添加一个。

参考：模拟 jQuery 的“include_once”

或者

include_once 等效于 js。参考：https : //raw.github.com/kvz/phpjs/master/functions/language/include_once.js

function include_once (filename) {
  // http://kevin.vanzonneveld.net
  // +   original by: Legaev Andrey
  // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   improved by: Michael White (http://getsprink.com)
  // +      input by: Brett Zamir (http://brett-zamir.me)
  // +   bugfixed by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  // +   bugfixed by: Brett Zamir (http://brett-zamir.me)
  // -    depends on: include
  // %        note 1: Uses global: php_js to keep track of included files (though private static variable in namespaced version)
  // *     example 1: include_once('http://www.phpjs.org/js/phpjs/_supporters/pj_test_supportfile_2.js');
  // *     returns 1: true
  var cur_file = {};
  cur_file[this.window.location.href] = 1;

  // BEGIN STATIC
  try { // We can't try to access on window, since it might not exist in some environments, and if we use "this.window"
    //    we risk adding another copy if different window objects are associated with the namespaced object
    php_js_shared; // Will be private static variable in namespaced version or global in non-namespaced
    //   version since we wish to share this across all instances
  } catch (e) {
    php_js_shared = {};
  }
  // END STATIC
  if (!php_js_shared.includes) {
    php_js_shared.includes = cur_file;
  }
  if (!php_js_shared.includes[filename]) {
    if (this.include(filename)) {
      return true;
    }
  } else {
    return true;
  }
  return false;
}

即使您可能附加了一个头部，它也可能不适用于所有浏览器。这是我发现始终如一地工作的唯一方法。

<script type="text/javascript">
if (typeof jQuery == 'undefined') {
  document.write('<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"><\/script>');        
  } 
</script>