Javascript 未定义变量：脚本中的窗口？

我有一个错误：

PHP通知–yii\base\ErrorException

未定义变量：窗口

函数在ewport（$el）{

我的部分代码：

<?php
$lazyload = <<< JS
$(function() {
  var $window = $(window),
      images = [],
      imagesToBeLoaded = 0,
      i,
      src;

  function throttle(func, wait) {
    var timeout;
    return function() {
      var context = this, args = arguments;
      if(!timeout) {
        timeout = setTimeout(function() {
          timeout = null;
        }, wait);
        func.apply(context, args);
      }
    };
  }

  function inViewport($el) {
    var top = $window.scrollTop(),
        left = $window.scrollLeft(),
        bottom = top + $window.height(),
        right = left + $window.width(),
        offset = $el.offset(),
        thisTop = offset.top,
        thisLeft = offset.left,
        thisBottom = thisTop + $el.outerHeight(),
        thisRight = thisLeft + $el.outerWidth();

                      ...

JS;
$this->registerJs($lazyload, View::POS_READY);
?>

---

引用herdoc的名称，以防止PHP将
$window
解释为变量：

<?php
$lazyload = <<< 'JS'
...
JS;

引用herdoc的名称，以防止PHP将
$window
解释为变量：

<?php
$lazyload = <<< 'JS'
...
JS;