如何使用同一脚本的多个实例终止正确的web worker

我正在使用web workers在网页中添加自定义数量的倒计时计时器。每个计时器创建计时器web worker的一个实例。一切正常。。。但我想在计时器等于“00:00”时终止web worker。当计时器过期时,以下代码将始终终止页面中显示的最后一个web worker。如何终止正确的一个?

$(document).ready(function() {
  $('.frequency-item').each(function(i, e) {
    children = $(e).children();
    observationLengthInMinutesId = children[1].id;
    timerId = children[3].id;
    startTimer(observationLengthInMinutesId, timerId);
  });
});

function startTimer(observationLengthInMinutesId, timerId) {
  w = null;

  if (typeof(Worker) !== "undefined") {
    if (w == null) {
      w = new Worker("/js/simple-timer.js");
      w.postMessage($('#' + observationLengthInMinutesId).val());
    }

    w.onmessage = function(event) {
      $('#' + timerId).text(event.data);

      //the problem is here    
      if (event.data == '00:00') {
        w.terminate();
      }

      /*
      I fixed in the following way but without terminating the workers! 
      I don't like it
      if($( '#' + timerId ).text() != '00:00') {
          $( '#' + timerId ).text(event.data);
      }
      */
    };
  } else {
    // Web workers are not supported by your browser
    $('#' + timerId).text("Sorry, your browser does not support Web Workers ...");
  }
}