How to convert UTF8 string to byte array

Question

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 lt str length  i          arr push str charCodeAt i

User · Answer

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  lt  utf8 length  i          arr push utf8 charCodeAt i

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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  lt  str length  index              var code   str charCodeAt index           if  code  lt   0x7F                value destIndex      code            else if  code  lt   0x7FF                value destIndex        code  gt  gt  6    amp  0x1F    0xC0              value destIndex        code  gt  gt  0    amp  0x3F    0x80            else if  code  lt   0xFFFF                value destIndex        code  gt  gt  12   amp  0x0F    0xE0              value destIndex        code  gt  gt  6    amp  0x3F    0x80              value destIndex        code  gt  gt  0    amp  0x3F    0x80            else if  code  lt   0x1FFFFF                value destIndex        code  gt  gt  18   amp  0x07    0xF0              value destIndex        code  gt  gt  12   amp  0x3F    0x80              value destIndex        code  gt  gt  6    amp  0x3F    0x80              value destIndex        code  gt  gt  0    amp  0x3F    0x80            else if  code  lt   0x03FFFFFF                value destIndex        code  gt  gt  24   amp  0x03    0xF0              value destIndex        code  gt  gt  18   amp  0x3F    0x80              value destIndex        code  gt  gt  12   amp  0x3F    0x80              value destIndex        code  gt  gt  6    amp  0x3F    0x80              value destIndex        code  gt  gt  0    amp  0x3F    0x80            else if  code  lt   0x7FFFFFFF                value destIndex        code  gt  gt  30   amp  0x01    0xFC              value destIndex        code  gt  gt  24   amp  0x3F    0x80              value destIndex        code  gt  gt  18   amp  0x3F    0x80              value destIndex        code  gt  gt  12   amp  0x3F    0x80              value destIndex        code  gt  gt  6    amp  0x3F    0x80              value destIndex        code  gt  gt  0    amp  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  lt  0            byteValue   byteValue  amp  0x00FF            var str   byteValue toString 2       var len   str length      var prefix           for  var i   len  i  lt  8  i              prefix     0             return prefix   str

User · Answer

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

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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

User · Answer

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  lt  str length  i          var c   str charCodeAt i       if  c  lt  128          out p      c        else if  c  lt  2048          out p       c  gt  gt  6    192        out p       c  amp  63    128        else if             c  amp  0xFC00     0xD800   amp  amp   i   1   lt  str length  amp  amp            str charCodeAt i   1   amp  0xFC00     0xDC00              Surrogate Pair       c   0x10000     c  amp  0x03FF   lt  lt  10     str charCodeAt   i   amp  0x03FF         out p       c  gt  gt  18    240        out p        c  gt  gt  12   amp  63    128        out p        c  gt  gt  6   amp  63    128        out p       c  amp  63    128        else         out p       c  gt  gt  12    224        out p        c  gt  gt  6   amp  63    128        out p       c  amp  63    128              return out

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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        gt           reader onload   event   gt                done new Uint8Array event target result   str              reader readAsArrayBuffer new Blob  str     type   application octet-stream            return   done  callback   gt    done   callback           Call it like this   stringToBytes   u 1f4a9    done bytes   gt        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

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function convertByte         var c document getElementById  quot str quot   value      var arr           var i 0      for var ind 0 ind lt c length ind                  arr ind  c charCodeAt i           i                  document getElementById  quot result quot   innerHTML  quot The converted value is  quot  arr join  quot  quot

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The TextEncoder and TextDecoder Encoding API will let you both encode and decode UTF-8 easily  using typed arrays   const encoded   new TextEncoder   encode  quot Ge a s     s  e quot    const decoded   new TextDecoder   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   Older versions of TextEncoder supported other different encodings as a string constructor parameter argument  but since Firefox 48 and Chrome 53 this is no-longer supported and TextEncoder and TextDecoder support only UTF-8  So this will no-longer work in modern web-browsers used since 2016  new TextDecoder  quot shift-jis quot   decode new Uint8Array textbuffer

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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  lt  str length  i              var charcode   str charCodeAt i           if  charcode  lt  0x80  utf8 push charcode           else if  charcode  lt  0x800                utf8 push 0xc0    charcode  gt  gt  6                          0x80    charcode  amp  0x3f                      else if  charcode  lt  0xd800    charcode  gt   0xe000                utf8 push 0xe0    charcode  gt  gt  12                          0x80     charcode gt  gt 6   amp  0x3f                          0x80    charcode  amp  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  amp  0x3ff  lt  lt 10                           str charCodeAt i   amp  0x3ff                utf8 push 0xf0    charcode  gt  gt 18                          0x80     charcode gt  gt 12   amp  0x3f                          0x80     charcode gt  gt 6   amp  0x3f                          0x80    charcode  amp  0x3f                        return utf8

[javascript] How to convert UTF8 string to byte array?

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