禁用backspace键的后退功能，但是可以删除文本内容

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function banBackSpace(e) {  <span style="white-space:pre"></span>var ev = e || window.event;//获取event对象  <span style="white-space:pre"></span>var obj = ev.target || ev.srcElement;//获取事件源  <span style="white-space:pre"></span>var t = obj.type || obj.getAttribute('type');//获取事件源类型  <span style="white-space:pre"></span>//获取作为判断条件的事件类型  <span style="white-space:pre"></span>var vReadOnly = obj.readOnly;  <span style="white-space:pre"></span>var vDisabled = obj.disabled;  <span style="white-space:pre"></span>//处理undefined值情况  <span style="white-space:pre"></span>vReadOnly = (vReadOnly == undefined) ? false : vReadOnly;  <span style="white-space:pre"></span>vDisabled = (vDisabled == undefined) ? true : vDisabled;  <span style="white-space:pre"></span>//当敲Backspace键时，事件源类型为密码或单行、多行文本的，  <span style="white-space:pre"></span>//并且readOnly属性为true或disabled属性为true的，则退格键失效  <span style="white-space:pre"></span>var flag1 = ev.keyCode == 8 && (t == "password" || t == "text" || t == "textarea") && (vReadOnly == true || vDisabled == true);  <span style="white-space:pre"></span>//当敲Backspace键时，事件源类型非密码或单行、多行文本的，则退格键失效  <span style="white-space:pre"></span>var flag2 = ev.keyCode == 8 && t != "password" && t != "text" && t != "textarea";  <span style="white-space:pre"></span>//判断  <span style="white-space:pre"></span>if (flag2 || flag1) return false  }  //禁止退格键 作用于Firefox、Opera  document.onkeypress = banBackSpace;  //禁止退格键 作用于IE、Chrome  document.onkeydown = banBackSpace;

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