how to block special characters in javascript

1var iChars = "!@#$%^&*()+=-[]\\\';,./{}|\":<>?";
2
3for (var i = 0; i < document.formname.fieldname.value.length; i++) {
4    if (iChars.indexOf(document.formname.fieldname.value.charAt(i)) != -1) {
5        alert ("Your username has special characters. \nThese are not allowed.\n Please remove them and try again.");
6        return false;
7    }
8}
9

1var iChars = "!@#$%^&*()+=-[]\\\';,./{}|\":<>?";
2
3for (var i = 0; i < document.formname.fieldname.value.length; i++) {
4    if (iChars.indexOf(document.formname.fieldname.value.charAt(i)) != -1) {
5        alert ("Your username has special characters. \nThese are not allowed.\n Please remove them and try again.");
6        return false;
7    }
8}