<?php
$this->registerJs('
$(function () {
//为所欲为的写你想要写的js代码吧
$......
});
', \yii\web\View::POS_END);

 $this->registerJsFile('http://example.com/js/main.js', ['depends' => [\yii\web\JqueryAsset::className()]]); 

 namespace backend\assets;
use yii\web\AssetBundle;
/**
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @since 2.0
 */
class AppAsset extends AssetBundle
{
  public $basePath = '@webroot';
  public $baseUrl = '@web';
  public $css = [
    'css/site.css',
  ];
  public $js = [
  ];
  public $depends = [
    'yii\web\YiiAsset',
    'yii\bootstrap\BootstrapAsset',
  ];  //定义按需加载JS方法，注意加载顺序在最后 
  public static function addScript($view, $jsfile) { 
    $view->registerJsFile($jsfile, [AppAsset::className(), 'depends' => 'backend\assets\AppAsset']); 
  } 
  //定义按需加载css方法，注意加载顺序在最后 
  public static function addCss($view, $cssfile) { 
    $view->registerCssFile($cssfile, [AppAsset::className(), 'depends' => 'backend\assets\AppAsset']); 
  } 
}

use backend\assets\AppAsset;
AppAsset::register($this);
AppAsset::addScript($this,'/css/main.js');

$this->registerJsFile('/css/main.js',['depends'=>['backend\assets\AppAsset'], 'position'=> $this::POS_END]);
$this->registerJsFile('/css/left.js',['depends'=>['backend\assets\AppAsset']]);
$this->registerJsFile('/css/extension.js',['depends'=>['backend\assets\AppAsset']]);