How to convert UTF8 string to byte array?

The .charCodeAt function returns with the unicode code of the caracter. But I would like to get the byte array instead. I know, if the charcode is over 127, then the character is stored in two or more bytes.

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

The logic of encoding Unicode in UTF-8 is basically:

• Up to 4 bytes per character can be used. The fewest number of bytes possible is used.
• Characters up to U+007F are encoded with a single byte.
• For multibyte sequences, the number of leading 1 bits in the first byte gives the number of bytes for the character. The rest of the bits of the first byte can be used to encode bits of the character.
• The continuation bytes begin with 10, and the other 6 bits encode bits of the character.

Here's a function I wrote a while back for encoding a JavaScript UTF-16 string in UTF-8:

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));
        }
        // surrogate pair
        else {
            i++;
            // UTF-16 encodes 0x10000-0x10FFFF by
            // subtracting 0x10000 and splitting the
            // 20 bits of 0x0-0xFFFFF into two halves
            charcode = 0x10000 + (((charcode & 0x3ff)<<10)
                      | (str.charCodeAt(i) & 0x3ff));
            utf8.push(0xf0 | (charcode >>18), 
                      0x80 | ((charcode>>12) & 0x3f), 
                      0x80 | ((charcode>>6) & 0x3f), 
                      0x80 | (charcode & 0x3f));
        }
    }
    return utf8;
}

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));
}

The Encoding API lets you both encode and decode UTF-8 easily (using typed arrays):

var encoded = new TextEncoder().encode("Γεια σου κόσμε");
var decoded = new TextDecoder("utf-8").decode(encoded);
    
console.log(encoded, decoded);

Browser support isn't too bad, and there's a polyfill that should work in IE11 and older versions of Edge.

While TextEncoder can only encode to UTF-8, TextDecoder supports other encodings. I used it to decode Japanese text (Shift-JIS) in this way:

// Shift-JIS encoded text; must be a byte array due to values 129 and 130.
var arr = [130, 108, 130, 102, 130, 80, 129,  64, 130, 102, 130,  96, 130, 108, 130, 100,
           129,  64, 130,  99, 130, 96, 130, 115, 130,  96, 129, 124, 130,  79, 130, 80];
// Convert to byte array
var data = new Uint8Array(arr);
// Decode with TextDecoder
var decoded = new TextDecoder("shift-jis").decode(data.buffer);
console.log(decoded);

The Google Closure library has functions to convert to/from UTF-8 and byte arrays. If you don't want to use the whole library, you can copy the functions from here. For completeness, the code to convert to a string to a UTF-8 byte array is:

goog.crypt.stringToUtf8ByteArray = function(str) {
  // TODO(user): Use native implementations if/when available
  var out = [], p = 0;
  for (var i = 0; i < str.length; i++) {
    var c = str.charCodeAt(i);
    if (c < 128) {
      out[p++] = c;
    } else if (c < 2048) {
      out[p++] = (c >> 6) | 192;
      out[p++] = (c & 63) | 128;
    } else if (
        ((c & 0xFC00) == 0xD800) && (i + 1) < str.length &&
        ((str.charCodeAt(i + 1) & 0xFC00) == 0xDC00)) {
      // Surrogate Pair
      c = 0x10000 + ((c & 0x03FF) << 10) + (str.charCodeAt(++i) & 0x03FF);
      out[p++] = (c >> 18) | 240;
      out[p++] = ((c >> 12) & 63) | 128;
      out[p++] = ((c >> 6) & 63) | 128;
      out[p++] = (c & 63) | 128;
    } else {
      out[p++] = (c >> 12) | 224;
      out[p++] = ((c >> 6) & 63) | 128;
      out[p++] = (c & 63) | 128;
    }
  }
  return out;
};

Assuming the question is about a DOMString as input and the goal is to get an Array, that when interpreted as string (e.g. written to a file on disk), would be UTF-8 encoded:

Now that nearly all modern browsers support Typed Arrays, it'd be ashamed if this approach is not listed:

• According to the W3C, software supporting the File API should accept DOMStrings in their Blob constructor (see also: String encoding when constructing a Blob)
• Blobs can be converted to an ArrayBuffer using the .readAsArrayBuffer() function of a File Reader
• Using a DataView or constructing a Typed Array with the buffer read by the File Reader, one can access every single byte of the ArrayBuffer

Example:

// Create a Blob with an Euro-char (U+20AC)
var b = new Blob(['€']);
var fr = new FileReader();

fr.onload = function() {
    ua = new Uint8Array(fr.result);
    // This will log "3|226|130|172"
    //                  E2  82  AC
    // In UTF-16, it would be only 2 bytes long
    console.log(
        fr.result.byteLength + '|' + 
        ua[0]  + '|' + 
        ua[1] + '|' + 
        ua[2] + ''
    );
};
fr.readAsArrayBuffer(b);

Play with that on JSFiddle. I haven't benchmarked this yet but I can imagine this being efficient for large DOMStrings as input.

You can save a string raw as is by using FileReader.

Save the string in a blob and call readAsArrayBuffer(). Then the onload-event results an arraybuffer, which can converted in a Uint8Array. Unfortunately this call is asynchronous.

This little function will help you:

function stringToBytes(str)
{
    let reader = new FileReader();
    let done = () => {};
    
    reader.onload = event =>
    {
        done(new Uint8Array(event.target.result), str);
    };
    reader.readAsArrayBuffer(new Blob([str], { type: "application/octet-stream" }));
    
    return { done: callback => { done = callback; } };
}

Call it like this:

stringToBytes("\u{1f4a9}").done(bytes =>
{
    console.log(bytes);
});

output: [240, 159, 146, 169]

explanation:

JavaScript use UTF-16 and surrogate-pairs to store unicode characters in memory. To save unicode character in raw binary byte streams an encoding is necessary. Usually and in the most case, UTF-8 is used for this. If you not use an enconding you can't save unicode character, just ASCII up to 0x7f.

FileReader.readAsArrayBuffer() uses UTF-8.

I was using Joni's solution and it worked fine, but this one is much shorter.

This was inspired by the atobUTF16() function of Solution #3 of Mozilla's Base64 Unicode discussion

function convertStringToUTF8ByteArray(str) {
    let binaryArray = new Uint8Array(str.length)
    Array.prototype.forEach.call(binaryArray, function (el, idx, arr) { arr[idx] = str.charCodeAt(idx) })
    return binaryArray
}

As there is no pure byte type in JavaScript we can represent a byte array as an array of numbers, where each number represents a byte and thus will have an integer value between 0 and 255 inclusive.

Here is a simple function that does convert a JavaScript string into an Array of numbers that contain the UTF-8 encoding of the string:

function toUtf8(str) {
    var value = [];
    var destIndex = 0;
    for (var index = 0; index < str.length; index++) {
        var code = str.charCodeAt(index);
        if (code <= 0x7F) {
            value[destIndex++] = code;
        } else if (code <= 0x7FF) {
            value[destIndex++] = ((code >> 6 ) & 0x1F) | 0xC0;
            value[destIndex++] = ((code >> 0 ) & 0x3F) | 0x80;
        } else if (code <= 0xFFFF) {
            value[destIndex++] = ((code >> 12) & 0x0F) | 0xE0;
            value[destIndex++] = ((code >> 6 ) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 0 ) & 0x3F) | 0x80;
        } else if (code <= 0x1FFFFF) {
            value[destIndex++] = ((code >> 18) & 0x07) | 0xF0;
            value[destIndex++] = ((code >> 12) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 6 ) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 0 ) & 0x3F) | 0x80;
        } else if (code <= 0x03FFFFFF) {
            value[destIndex++] = ((code >> 24) & 0x03) | 0xF0;
            value[destIndex++] = ((code >> 18) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 12) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 6 ) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 0 ) & 0x3F) | 0x80;
        } else if (code <= 0x7FFFFFFF) {
            value[destIndex++] = ((code >> 30) & 0x01) | 0xFC;
            value[destIndex++] = ((code >> 24) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 18) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 12) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 6 ) & 0x3F) | 0x80;
            value[destIndex++] = ((code >> 0 ) & 0x3F) | 0x80;
        } else {
            throw new Error("Unsupported Unicode character \"" 
                + str.charAt(index) + "\" with code " + code + " (binary: " 
                + toBinary(code) + ") at index " + index
                + ". Cannot represent it as UTF-8 byte sequence.");
        }
    }
    return value;
}

function toBinary(byteValue) {
    if (byteValue < 0) {
        byteValue = byteValue & 0x00FF;
    }
    var str = byteValue.toString(2);
    var len = str.length;
    var prefix = "";
    for (var i = len; i < 8; i++) {
        prefix += "0";
    }
    return prefix + str;
}

function convertByte()
{
    var c=document.getElementById("str").value;
    var arr = [];
    var i=0;
    for(var ind=0;ind<c.length;ind++)
    {
        arr[ind]=c.charCodeAt(i);
        i++;
    }    
    document.getElementById("result").innerHTML="The converted value is "+arr.join("");    
}