简洁版本
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指出了反射的属性，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
你必须经历element.getAttribute和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”，您将其错误地设置为String对象，而不是其所需的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（用一种类型的语言表示）是：
HtmlAttribute attribute = new HtmlAttribute();
attribute.value = "";
element.attributes["required"] = attribute;
这就是为什么：
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;
什么，你不想要做的就是误用.attributes集合：
element.attributes.required = true; //WRONG!
if (element.attributes.required) {...} //WRONG!
element.attributes.required = false; //WRONG!
测试案例 这导致围绕required属性的使用进行测试，比较通过属性返回的值和反映的属性
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)
尝试.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]
这解释了为什么您不应该.attributes直接与收款人交谈。您不是在操纵属性的值，而是操纵代表属性本身的对象。
设置required属性的正确方法是什么？您有两种选择，即体现属性或通过正确设置属性：
element.setAttribute("required", ""); //Correct
edName.required = true; //Correct
严格来说，任何其他值都会“设置”属性。但是Boolean属性的定义规定应仅将其设置为空字符串""以表示true。下面的方法都工作到设定的required 布尔属性，
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
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？ 尝试删除该required属性的诀窍是很容易意外地将其打开：
edName.removeAttribute("required"); //Correct
edName.required = false; //Correct
用无效的方法：
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，还可以使用任何“ falsey”值将其关闭，而使用trues值将其打开。但是为了清楚起见，请坚持对与错。
通过以下.hasAttribute("required")方法检查属性的存在：
if (edName.hasAttribute("required"))
{
}
您也可以通过Boolean反映的.required属性进行检查 ：
if (edName.required)
{
}
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