Concurrent Upload of Large Files in JavaScript

calcFileMD5.js

function calcFileMD5(file) {
  return new Promise((resolve, reject) => {
    let chunkSize = 2097152, // 2M
      chunks = Math.ceil(file.size / chunkSize),
      currentChunk = 0,
      spark = new SparkMD5.ArrayBuffer(),
      fileReader = new FileReader();

    fileReader.onload = (e) => {
      spark.append(e.target.result);
      currentChunk++;
      if (currentChunk < chunks) {
        loadNext();
      } else {
        resolve(spark.end());
      }
    };

    fileReader.onerror = (e) => {
      reject(fileReader.error);
      reader.abort();
    };

    function loadNext() {
      let start = currentChunk * chunkSize,
        end = start + chunkSize >= file.size ? file.size : start + chunkSize;
      fileReader.readAsArrayBuffer(file.slice(start, end));
    }
    loadNext();
  });
}