Javascript, spliced FileReader for large files with Promises, how?

Question

I\'m trying to use FileReader to read several files sequentially using promises for that. The problem is that I need to divide the reading operations in several chunks to make t

Answer 1

Your upload() function does not always return a Promise. It does in the else condition, but the if condition doesn't. You have a return statement, but it's inside of a callback, so it won't be received by the caller of upload.

Try changing that to this:

function upload() {
  if (start < end) {
    return new Promise(function (resolve, reject) {
      var chunk = uploading_file.slice(start, temp_end);
      var reader = new FileReader();
      reader.readAsArrayBuffer(chunk);
      reader.onload = function(e) {
        if (e.target.readyState == 2) {
          content += new TextDecoder("utf-8").decode(e.target.result);
          start = temp_end;
          temp_end = start + BYTES_PER_CHUNK;
          if (temp_end > end) temp_end = end;
          resolve(upload());
        }
      }
    });
  } else {
    uploading_file = null;
    console.log(content); // it shows the content of the file
    return Promise.resolve(content);
  }
}

Now, upload always returns a Promise instead of undefined.