fetch 獲取文件上傳過程

https://jakearchibald.com/2016/streams-ftw/ `

// fetch() returns a promise that
// resolves once headers have been received

fetch(url).then((response) => {
  // response.body is a readable stream.
  // Calling getReader() gives us exclusive access to
  // the stream's content
  var reader = response.body.getReader();
  var bytesReceived = 0;

  // read() returns a promise that resolves
  // when a value has been received
  return reader.read().then(function processResult(result) {
	// Result objects contain two properties:
	// done  - true if the stream has already given
	//         you all its data.
	// value - some data. Always undefined when
	//         done is true.
	if (result.done) {
	  console.log('Fetch complete');
	  return;
	}

	// result.value for fetch streams is a Uint8Array
	bytesReceived += result.value.length;
	console.log('Received', bytesReceived, 'bytes of data so far');

	// Read some more, and call this function again
	return reader.read().then(processResult);
  });
});

`