10 声明字符串"javascript学习笔记,String字符串相关!" ( 追加和替换这两种用法做不出)

<script language="javascript">
<!--
// 声明字符串"javascript学习笔记,String字符串相关!"
var txt = "javascript学习笔记,String字符串相关!"
document.write( "<br>" );

// 取得字符串长度
document.write(txt.length);
document.write( "<br>" );

//查找"记"字所在位置
function IndexDemo(txt2)
{
var s = txt.indexOf(txt2);
return(s);
}
document.write(IndexDemo("记"));
document.write( "<br>" );

//第12个字符是什么
sub1 = txt.charAt(12)
document.write(sub1);
document.write( "<br>" );

//截取子字符串"学习笔记"
sub2 = txt.substring(10,14);
document.write(sub2);
document.write( "<br>" );

-->
</script>