Reading bytes from a JavaScript string

Question

I have a string containing binary data in JavaScript. Now I want to read, for example, an integer from it. So I get the first 4 characters, use charCodeAt, do some shifting, etc. to get an integer.

The problem is that strings in JavaScript are UTF-16 (instead of ASCII) and charCodeAt often returns values higher than 256.

The Mozilla reference states that "The first 128 Unicode code points are a direct match of the ASCII character encoding." (what about ASCII values > 128?).

How can I convert the result of charCodeAt to an ASCII value? Or is there a better way to convert a string of four characters to a 4 byte integer?

Answer 1

I believe that you can can do this with relatively simple bit operations:

function stringToBytes ( str ) {
  var ch, st, re = [];
  for (var i = 0; i < str.length; i++ ) {
    ch = str.charCodeAt(i);  // get char 
    st = [];                 // set up "stack"
    do {
      st.push( ch & 0xFF );  // push byte to stack
      ch = ch >> 8;          // shift value down by 1 byte
    }  
    while ( ch );
    // add stack contents to result
    // done because chars have "wrong" endianness
    re = re.concat( st.reverse() );
  }
  // return an array of bytes
  return re;
}

stringToBytes( "Au1242Bu4123C" );  // [65, 18, 66, 66, 65, 35, 67]

It should be a simple matter to sum the output up by reading the byte array as if it were memory and adding it up into larger numbers:

function getIntAt ( arr, offs ) {
  return (arr[offs+0] << 24) +
         (arr[offs+1] << 16) +
         (arr[offs+2] << 8) +
          arr[offs+3];
}

function getWordAt ( arr, offs ) {
  return (arr[offs+0] << 8) +
          arr[offs+1];
}

'\u' + getWordAt( stringToBytes( "Au1242" ), 1 ).toString(16);  // "1242"