JavaScript Strings are stored in UTF-16. To get UTF-8, you'll have to convert the String yourself.

One way is to mix encodeURIComponent(), which will output UTF-8 bytes URL-encoded, with unescape, as mentioned on ecmanaut.

var utf8 = unescape(encodeURIComponent(str));

var arr = [];
for (var i = 0; i < utf8.length; i++) {
    arr.push(utf8.charCodeAt(i));
}

You can use this function (gist):

function toUTF8Array(str) {
    var utf8 = [];
    for (var i=0; i < str.length; i++) {
        var charcode = str.charCodeAt(i);
        if (charcode < 0x80) utf8.push(charcode);
        else if (charcode < 0x800) {
            utf8.push(0xc0 | (charcode >> 6), 
                      0x80 | (charcode & 0x3f));
        }
        else if (charcode < 0xd800 || charcode >= 0xe000) {
            utf8.push(0xe0 | (charcode >> 12), 
                      0x80 | ((charcode>>6) & 0x3f), 
                      0x80 | (charcode & 0x3f));
        }
        else {
            // let's keep things simple and only handle chars up to U+FFFF...
            utf8.push(0xef, 0xbf, 0xbd); // U+FFFE "replacement character"
        }
    }
    return utf8;
}

Example of use:

>>> toUTF8Array("中€")
[228, 184, 173, 226, 130, 172]

If you want negative numbers for values over 127, like Java's byte-to-int conversion does, you have to tweak the constants and use

            utf8.push(0xffffffc0 | (charcode >> 6), 
                      0xffffff80 | (charcode & 0x3f));

and

            utf8.push(0xffffffe0 | (charcode >> 12), 
                      0xffffff80 | ((charcode>>6) & 0x3f), 
                      0xffffff80 | (charcode & 0x3f));

JavaScript has no concept of character encoding for String, everything is in UTF-16. Most of time time the value of a char in UTF-16 matches UTF-8, so you can forget it's any different.

There are more optimal ways to do this but

function s(x) {return x.charCodeAt(0);}
"test.message".split('').map(s);
// [116, 101, 115, 116, 46, 109, 101, 115, 115, 97, 103, 101]

So what is unescape(encodeURIComponent(str)) doing? Let's look at each individually,

1. encodeURIComponent is converting every character in str which is illegal or has a meaning in URI Syntax into a URI escaped version so that there is no problem using it as a key or value in the search component of a URI, for example encodeURIComponent('&='); // "%26%3D" Notice how this is now a 6 character long String.
2. unescape is actually depreciated, but it does a similar job to decodeURI or decodeURIComponent (the reverse of encodeURIComponent). If we look in the ES5 spec we can see 11. Let c be the character whose code unit value is the integer represented by the four hexadecimal digits at positions k+2, k+3, k+4, and k+5 within Result(1).
   So, 4 digits is 2 bytes is "UTF-8", however as I mentioned, all Strings are UTF-16, so it's really a UTF-16 string limiting itself to UTF-8.