Java String.getBytes(“UTF8”) JavaScript analog

You don't need to write a full-on UTF-8 encoder; there is a much easier JS idiom to convert a Unicode string into a string of bytes representing UTF-8 code units:

unescape(encodeURIComponent(str))

(This works because the odd encoding used by escape/unescape uses %xx hex sequences to represent ISO-8859-1 characters with that code, instead of UTF-8 as used by URI-component escaping. Similarly decodeURIComponent(escape(bytes)) goes in the other direction.)

So if you want an Array out it would be:

function toUTF8Array(str) {
    var utf8= unescape(encodeURIComponent(str));
    var arr= new Array(utf8.length);
    for (var i= 0; i<utf8.length; i++)
        arr[i]= utf8.charCodeAt(i);
    return arr;
}

The following function will deal with those above U+FFFF.

Because javascript text are in UTF-16, two "characters" are used in a string to represent a character above BMP, and charCodeAt returns the corresponding surrogate code. The fixedCharCodeAt handles this.

function encodeTextToUtf8(text) {
    var bin = [];
    for (var i = 0; i < text.length; i++) {
        var v = fixedCharCodeAt(text, i);
        if (v === false) continue;
        encodeCharCodeToUtf8(v, bin);
    }
    return bin;
}

function encodeCharCodeToUtf8(codePt, bin) {
    if (codePt <= 0x7F) {
        bin.push(codePt);
    } else if (codePt <= 0x7FF) {
        bin.push(192 | (codePt >> 6), 128 | (codePt & 63));
    } else if (codePt <= 0xFFFF) {
        bin.push(224 | (codePt >> 12),
            128 | ((codePt >> 6) & 63),
            128 | (codePt & 63));
    } else if (codePt <= 0x1FFFFF) {
        bin.push(240 | (codePt >> 18),
            128 | ((codePt >> 12) & 63), 
            128 | ((codePt >> 6) & 63),
            128 | (codePt & 63));
    }
}

function fixedCharCodeAt (str, idx) {  
    // ex. fixedCharCodeAt ('\uD800\uDC00', 0); // 65536  
    // ex. fixedCharCodeAt ('\uD800\uDC00', 1); // 65536  
    idx = idx || 0;  
    var code = str.charCodeAt(idx);  
    var hi, low;  
    if (0xD800 <= code && code <= 0xDBFF) { // High surrogate (could change last hex to 0xDB7F to treat high private surrogates as single characters)  
        hi = code;  
        low = str.charCodeAt(idx+1);  
        if (isNaN(low)) {  
            throw(encoding_error.invalid_surrogate_pair.replace('%pos%', idx));
        }  
        return ((hi - 0xD800) * 0x400) + (low - 0xDC00) + 0x10000;  
    }  
    if (0xDC00 <= code && code <= 0xDFFF) { // Low surrogate  
        // We return false to allow loops to skip this iteration since should have already handled high surrogate above in the previous iteration  
        return false;  
        /*hi = str.charCodeAt(idx-1); 
          low = code; 
          return ((hi - 0xD800) * 0x400) + (low - 0xDC00) + 0x10000;*/  
    }  
    return code;  
}  

TextEncoder is part of the Encoding Living Standard and according to the Encoding API entry from the Chromium Dashboard, it shipped in Firefox and will ship in Chrome 38. There is also a text-encoding polyfill available for other browsers.

The JavaScript code sample below returns a Uint8Array filled with the values you expect.

(new TextEncoder()).encode("string") 
// [115, 116, 114, 105, 110, 103]

A more interesting example that betters shows UTF-8 replaces the in in string with îñ:

(new TextEncoder()).encode("strîñg")
[115, 116, 114, 195, 174, 195, 177, 103]