JS detects if the username contains Chinese characters

Web effects detect whether the user name contains Chinese characters
This verification code is more suitable for user registration verification, it can verify that the user entered the registered username that we do not want to appear in the string or Chinese characters, such as the following I defined letters as letters and numbers and _ lines, so that you can verify that the user submitted the database tutorial does not contain Chinese characters.
*/

function Checkifenglish (String)
{
var letters = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz1234567890_";
var i;
var C;
for (i = 0; i < string.length i + +)
{
c = String.charat (i);
if (Letters.indexof (c) < 0)
return false;
}
return true;
}

var str = ' 122 China ';
var str1 = ' FDAFSA_456776111CN ';
if (Checkifenglish (str))
{
Alert (' lawful username ');
}
Else
{
Alert (' String contains Chinese ');
}

If (Checkifenglish (str1))
{
 alert (' Legal user name ');
}
Else
{
 alert (contains Chinese in string);
}