我想在此代码中的计数器之前添加一个美元符号货币符号

var a = 0;
$(window).scroll(function() {

  var oTop = $('#counter').offset().top - window.innerHeight;
  if (a == 0 && $(window).scrollTop() > oTop) {
    $('.counter-value').each(function() {
      var $this = number_format(this),
        countTo = $this.attr('data-count');
      $({
        countNum: $this.text()
      }).animate({
        countNum: countTo
      }, {
        duration: 2000,
        easing: 'swing',
        step: function() {
          $this.text(Math.floor(this.countNum));
        },
        complete: function() {
          $this.text(this.countNum);
        }
      });
    });
    a = 1;
  }
});

function number_format(number) {
  var num = number.toLocaleString('en-US', {
    maximumFractionDigits: 0
  })
  return num;
}

var a = 0;
$(window).ready(function() {

  var oTop = $('#counter').offset().top - window.innerHeight;
  if (a == 0 && $(window).scrollTop() > oTop) {
    $('.counter-value').each(function() {
      var $this = $(this),
        countTo = $this.attr('data-count');
      $({
        countNum: $this.text()
      }).animate({
        countNum: countTo
      }, {
        duration: 2000,
        easing: 'swing',
        step: function() {
          $this.text(number_format(this.countNum));
        },
        complete: function() {
          $this.text(number_format(this.countNum));
        }
      });
    });
    a = 1;

  }
});

.counter-value {
  font-size: 80px;
  line-height: 2em;
  text-align: center;
  padding: 17px 0;
}

.counter-value:before {
  content: "$";
}

.counter-value:after {
  content: attr(data-desc);
  display: block;
  text-transform: none;
  font-size: 16px;
  line-height: 1.2em;
}

<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js">
</script>
<div id="counter">
  <div class="sqs-col sqs-col-4 counter-value" data-count="4172" data-desc="The average American’s 401(k) balance is $104,300 — 
    this amount of money can only be expected to generate $4,172 in 
    sustainable annual retirement income">0</div>
  <div class="sqs-col sqs-col-4 counter-value" data-count="1409" data-desc="As of March 2018, the average monthly Social Security 
    payment is $1,409">0</div>
  <div class="sqs-col sqs-col-4 counter-value" data-count="40000" data-desc="To live on $40,000 a year after stopping work, you will 
    need savings of about $1.18 million to support a 30-year 
    retirement">0</div>
</div>



<div style="border: 0px solid black">
</div>