切换焦点

<form action="#" id="form">
  <label class="" for=""
    >请输入电话号码 (格式：区号 -电话号码 -分机号 )
    <input type="text" name="tel1" size="3" maxlength="3" />
    <input type="text" name="tel2" size="8" maxlength="8" />
    <input type="text" name="tel3" size="3" maxlength="3" />
  </label>
</form>
<script>
  var form = document.querySelector('form');
  var tel1 = form.elements[0];
  var tel2 = form.elements[1];
  var tel3 = form.elements[2];
  tel1.focus();
  addHandler(tel1, 'keyup', autofocus);
  addHandler(tel2, 'keyup', autofocus);
  addHandler(tel3, 'keyup', autofocus);
  function addHandler(element, type, handle) {
    if (element.addEventListener) {
      element.addEventListener(type, handle, false);
    } else if (element.attachEvent) {
      element.attachEvent('on' + type, handle);
    } else {
      element['on' + type] = handle;
    }
  }
  var autofocus = function (event) {
    var e = event || window.event;
    var t = e.target || e.srcElement;
    if (t.value.length == t.maxLength) {
      var form = t.form;
      for (let i = 0, len = form.elements.length; i < len; i++) {
        if (form.elements[i] == t) {
          if (form.elements[i + 1]) {
            form.elements[i + 1].focus();
          }
          return;
        }
      }
    }
  };
</script>