精简版
element.setAttribute("required", ""); //turns required on
element.required = true; //turns required on through reflected attribute
jQuery(element).attr('required', ''); //turns required on
$("#elementId").attr('required', ''); //turns required on
element.removeAttribute("required"); //turns required off
element.required = false; //turns required off through reflected attribute
jQuery(element).removeAttr('required'); //turns required off
$("#elementId").removeAttr('required'); //turns required off
if (edName.hasAttribute("required")) { } //check if required
if (edName.required) { } //check if required using reflected attribute
长版
一旦 TJ Crowder 设法指出反射属性，我就知道以下语法是错误的：
element.attributes["name"] = value; //bad! Overwrites the HtmlAttribute object
element.attributes.name = value; //bad! Overwrites the HtmlAttribute object
value = element.attributes.name; //bad! Returns the HtmlAttribute object, not its value
value = element.attributes["name"]; //bad! Returns the HtmlAttribute object, not its value
您必须通过element.getAttribute并且element.setAttribute：
element.getAttribute("foo"); //correct
element.setAttribute("foo", "test"); //correct
这是因为该属性实际上包含一个特殊的HtmlAttribute对象：
element.attributes["foo"]; //returns HtmlAttribute object, not the value of the attribute
element.attributes.foo; //returns HtmlAttribute object, not the value of the attribute
通过将属性值设置为“true”，您错误地将其设置为String对象，而不是它所需的HtmlAttribute对象：
element.attributes["foo"] = "true"; //error because "true" is not a HtmlAttribute object
element.setAttribute("foo", "true"); //error because "true" is not an HtmlAttribute object
从概念上讲，正确的想法（用类型语言表达）是：
HtmlAttribute attribute = new HtmlAttribute();
attribute.value = "";
element.attributes["required"] = attribute;
这就是为什么：
getAttribute(name)setAttribute(name, value)
存在。他们负责将值分配给内部的 HtmlAttribute 对象。
除此之外，还反映了一些属性。这意味着您可以从 Javascript 更好地访问它们：
//Set the required attribute
//element.setAttribute("required", "");
element.required = true;
//Check the attribute
//if (element.getAttribute("required")) {...}
if (element.required) {...}
//Remove the required attribute
//element.removeAttribute("required");
element.required = false;
您不想做的是错误地使用.attributes集合：
element.attributes.required = true; //WRONG!
if (element.attributes.required) {...} //WRONG!
element.attributes.required = false; //WRONG!
测试用例
这导致围绕属性的使用进行测试required，比较通过属性返回的值和反射的属性
document.getElementById("name").required;
document.getElementById("name").getAttribute("required");
结果：
HTML .required .getAttribute("required")
========================== =============== =========================
<input> false (Boolean) null (Object)
<input required> true (Boolean) "" (String)
<input required=""> true (Boolean) "" (String)
<input required="required"> true (Boolean) "required" (String)
<input required="true"> true (Boolean) "true" (String)
<input required="false"> true (Boolean) "false" (String)
<input required="0"> true (Boolean) "0" (String)
试图.attributes直接访问集合是错误的。它返回代表 DOM 属性的对象：
edName.attributes["required"] => [object Attr]
edName.attributes.required => [object Attr]
这就解释了为什么你永远不应该.attributes直接与收集者交谈。您不是在操作属性的值，而是在操作代表属性本身的对象。
需要怎么设置？
required设置属性的正确方法是什么？您有两个选择，要么是反射属性，要么是通过正确设置属性：
element.setAttribute("required", ""); //Correct
edName.required = true; //Correct
严格来说，任何其他值都会“设置”该属性。但是Boolean属性的定义规定它应该只设置为空字符串""来表示true。以下方法都可以设置布尔属性， required
但不要使用它们：
element.setAttribute("required", "required"); //valid, but not preferred
element.setAttribute("required", "foo"); //works, but silly
element.setAttribute("required", "true"); //Works, but don't do it, because:
element.setAttribute("required", "false"); //also sets required boolean to true
element.setAttribute("required", false); //also sets required boolean to true
element.setAttribute("required", 0); //also sets required boolean to true
我们已经知道尝试直接设置属性是错误的：
edName.attributes["required"] = true; //wrong
edName.attributes["required"] = ""; //wrong
edName.attributes["required"] = "required"; //wrong
edName.attributes.required = true; //wrong
edName.attributes.required = ""; //wrong
edName.attributes.required = "required"; //wrong
如何清除必填项？
尝试删除属性时的技巧required是很容易意外打开它：
edName.removeAttribute("required"); //Correct
edName.required = false; //Correct
使用无效的方式：
edName.setAttribute("required", null); //WRONG! Actually turns required on!
edName.setAttribute("required", ""); //WRONG! Actually turns required on!
edName.setAttribute("required", "false"); //WRONG! Actually turns required on!
edName.setAttribute("required", false); //WRONG! Actually turns required on!
edName.setAttribute("required", 0); //WRONG! Actually turns required on!
使用反射.required属性时，您还可以使用任何“假”值将其关闭，并使用真值将其打开。但为了清楚起见，只要坚持真假。
如何检查required？_
通过方法检查属性是否存在.hasAttribute("required")：
if (edName.hasAttribute("required"))
{
}
您还可以通过 布尔反射.required属性检查它：
if (edName.required)
{
}
