Starting the large file upload using chunks

Step6-LargeFileUpload-Start.js

//Helper method - depending on what chunk of data we are dealing with, we need to use the correct REST method...
function getUploadMethod(offset, length, total) {
  if (offset + length + 1 > total) {
    return 'finishupload';
  } else if (offset === 0) {
    return 'startupload';
  } else if (offset < total) {
    return 'continueupload';
  }

  return null;
}

module.exports = {
  upload: (file) => {
    return new Promise((resolve, reject) => {
      let ctx = jsomAppContext();
 
      //we have already done this bit...
      createDummaryFile(ctx, file.name, 'Documents').then(result => {
        let fr = new FileReader();
        let offset = 0;
        // the total file size in bytes...
        let total = file.size;
        // 1MB Chunks as represented in bytes (if the file is less than a MB, seperate it into two chunks of 80% and 20% the size)...
        let length = 1000000 > total ? total * 0.8 : 1000000;
        let chunks = [];

        fr.onload = evt => {
          while (offset < total) {
            //if we are dealing with the final chunk, we need to know...
            if (offset + length > total) {
              length = total - offset;
            }

            //work out the chunks that need to be processed and the associated REST method (start, continue or finish)
            chunks.push({ offset, length, method: getUploadMethod(offset, length, total) });

            offset += length;
          }

          //each chunk is worth a percentage of the total size of the file...
          const chunkPercentage = parseFloat(((total / chunks.length) / total)) * 100;

          if (chunks.length > 0) {
            //the unique guid identifier to be used throughout the upload session
            const id = utils.getGuid();

            //Start the upload - send the data to SP (we will show the uploadFile function in the next step)...
            uploadFile(evt.target.result, id, 'Documents', file.name, chunks, 0, 0, chunkPercentage, resolve, reject);
          }
        };

        //reads in the file using the fileReader HTML5 API (as an ArrayBuffer) - readAsBinaryString is not available in IE!
        fr.readAsArrayBuffer(file);
      });
    });
  }
};