function d2h(d) {
    return d.toString(16);
}
function h2d (h) {
    return parseInt(h, 16);
}
function stringToHex (tmp) {
    var str = '',
        i = 0,
        tmp_len = tmp.length,
        c;
    
    for (; i < tmp_len; i += 1) {
        c = tmp.charCodeAt(i);
        str += d2h(c) + ' ';
    }
    return str;
}
function hexToString (tmp) {
    var arr = tmp.split(' '),
        str = '',
        i = 0,
        arr_len = arr.length,
        c;
    
    for (; i < arr_len; i += 1) {
        c = String.fromCharCode( h2d( arr[i] ) );
        str += c;
    }
    
    return str;
}

String / Hex conversion

JavaScript