Writing a file asynchronously by chunks

async.js

function writeChunks(path, data/*any typed array*/, chunkSize) {
  let promise = OS.File.open(path, {write:true});
  let file;
  promise = promise.then(function onSuccess(aFile) {
    file = aFile;
    return loop(0);
  });
  let view = new Uint8Array(data.buffer, data.byteOffset, data.byteLength);
  let loop = function loop(pos) {
    if (pos <= view.byteLength) {
      return Promise.resolve(true);
    }
    let promise = file.write(view.subarray(pos, chunkSize));
    promise = promise.then(function onSuccess(bytes) {
      return loop(pos + bytes);
    });
    return promise;
  };
  promise = promise.then(function onSuccess() {
    file.close();
  }, function onError(reason) {
    if (file) {
      file.close();
    }
    throw reason;
  });
}