jQuery:当获取元素的值时,我需要在文档中声明变量吗?

我建议 不 这样做。如果您想更快/更方便地访问,请编写一个函数:

var getRoleId = (function() {
  var roleField = $('#RegistrationRole_ID').get(0);
  return function() {
    return roleField.value;
  };
})();

  if (getRoldId() == 22) { // whatever }

真正地 已经 (本质上)一个“全局变量”。该函数只是确保您没有执行“冗余”文档.getElementById()”调用(通过jQuery),但它保持了诚实,并避免了脚本“缓存”值不同步的潜在问题。

对。但是使用 var 只有一次,因为它声明了变量。

var selectedRole; // selectedRole is now global.
function one() {
    // If one() is called after doc ready
    // selectedRole will be $('#RegistrationRole_ID').val();
    // in it.
}
function two() {
    // If two() is called after doc ready
    // selectedRole will be $('#RegistrationRole_ID').val();
    // in it.
}
$(function() {
    selectedRole = $('#RegistrationRole_ID').val();  // This change that global
    // Now you can use one() and two() w/o redoing the same jQuery
    // over and over again.
    one(); two();
});

(function() {
    var selectedRole; // selectRole is now available everywhere in the anon fun()
                      //   but it is not global
    function one() {
        ...
    }
    function two() {
        ...
    }

    $(function() {
        selectedRole = $('#RegistrationRole_ID').val();  // Change the variable
        // Now you can use one() and two() and
        // selectedRole will be available:

        one(); two()
    });
}());