JavaScript & regex : How do I check if the string is ASCII only?

All you need to do it test that the characters are in the right character range.

function isASCII(str) {
    return /^[\x00-\x7F]*$/.test(str);
}

Or if you want to possibly use the extended ASCII character set:

function isASCII(str, extended) {
    return (extended ? /^[\x00-\xFF]*$/ : /^[\x00-\x7F]*$/).test(str);
}

You don't need a RegEx to do it, just check if all characters in that string have a char code between 0 and 127:

function isValid(str){
    if(typeof(str)!=='string'){
        return false;
    }
    for(var i=0;i<str.length;i++){
        if(str.charCodeAt(i)>127){
            return false;
        }
    }
    return true;
}