来源：https://www.cnblogs.com/attitudeY/p/6279985.html

BUG描述：$ is not defined 没有加载jquery成功

原因：Yii2.0将JS代码默认加载页面加载后

解决方案：

第一种方案：最简单方法是在 assets\AppAsset.php 中加上，页面前加载

public $jsOptions = array(

    'position' => \yii\web\View::POS_HEAD

);

第二种方案：But in production you usually want the scripts to load last, and instead you let Yii2 handle your javascript:

$this->registerJs(

    '$("document").ready(function(){ alert("hi"); });'

);

Now Yii will handle this js and place it after anything important (like jQuery).

You'll however soon notice that IDE's are usually bad at handling this kind of language nesting (JavaScript inside PHP) so the syntax highlighting is likely to break. One way to solve it is to register your script in a separate file:

$this->registerJsFile( 'myScript.js' );

If you want even more control about which order to load your scripts, you can add dependencies as your second argument, and additional options as the third:

$this->registerJsFile(

    'myScript.js',

    ['\backend\assets\AppAsset'],

    ['position' => '\yii\web\View::POS_END']

);

If you for some reason absolutely want the script to be rendered inline you can do:

$this->registerJs( $this->renderPartial('myScript.js') );