对js中startWith和endWith的扩展

<input name="searchContent" id="searchContent" type="text" onclick="test();">
<SCRIPT type="text/javascript" language="JavaScript">
function test(){
 var a  = "abcdef".startWith("abc");
 alert("a : "+a);
 var b  = "abcdef".endWith("def");
 alert("b : "+b);
}
String.prototype.endWith=function(str){
if(str==null||str==""||this.length==0||str.length>this.length)
  return false;
if(this.substring(this.length-str.length)==str)
  return true;
else
  return false;
return true;
}

String.prototype.startWith=function(str){
if(str==null||str==""||this.length==0||str.length>this.length)
  return false;
if(this.substr(0,str.length)==str)
  return true;
else
  return false;
return true;
}

</SCRIPT>