31 Mayıs 2019

Fetch: Download progress

The fetch method allows to track download progress.

Please note: there’s currently no way for fetch to track upload progress. For that purpose, please use XMLHttpRequest.

To track download progress, we can use response.body property. It’s a “readable stream” – a special object that provides body chunk-by-chunk, as it comes, so we can see how much is available at the moment.

Here’s the sketch of code that uses it to read response:

// instead of response.json() and other methods
const reader = response.body.getReader();

// infinite loop while the body is downloading
while(true) {
  // done is true for the last chunk
  // value is Uint8Array of the chunk bytes
  const {done, value} = await reader.read();

  if (done) {

  console.log(`Received ${value.length} bytes`)

So, we loop, while await reader.read() returns response chunks.

A chunk has two properties:

  • done – true when the reading is complete.
  • value – a typed array of bytes: Uint8Array.

To log the progress, we just need to count chunks.

Here’s the full code to get response and log the progress, more explanations follow:

// Step 1: start the fetch and obtain a reader
let response = await fetch('https://api.github.com/repos/javascript-tutorial/en.javascript.info/commits?per_page=100');

const reader = response.body.getReader();

// Step 2: get total length
const contentLength = +response.headers.get('Content-Length');

// Step 3: read the data
let receivedLength = 0; // length at the moment
let chunks = []; // array of received binary chunks (comprises the body)
while(true) {
  const {done, value} = await reader.read();

  if (done) {

  receivedLength += value.length;

  console.log(`Received ${receivedLength} of ${contentLength}`)

// Step 4: concatenate chunks into single Uint8Array
let chunksAll = new Uint8Array(receivedLength); // (4.1)
let position = 0;
for(let chunk of chunks) {
  chunksAll.set(chunk, position); // (4.2)
  position += chunk.length;

// Step 5: decode into a string
let result = new TextDecoder("utf-8").decode(chunksAll);

// We're done!
let commits = JSON.parse(result);

Let’s explain that step-by-step:

  1. We perform fetch as usual, but instead of calling response.json(), we obtain a stream reader response.body.getReader().

    Please note, we can’t use both these methods to read the same response. Either use a reader or a response method to get the result.

  2. Prior to reading, we can figure out the full response length from the Content-Length header.

    It may be absent for cross-domain requests (see chapter Fetch: Cross-Origin Requests) and, well, technically a server doesn’t have to set it. But usually it’s at place.

  3. Call await reader.read() until it’s done.

    We gather response chunks in the array. That’s important, because after the response is consumed, we won’t be able to “re-read” it using response.json() or another way (you can try, there’ll be an error).

  4. At the end, we have chunks – an array of Uint8Array byte chunks. We need to join them into a single result. Unfortunately, there’s no single method that concatenates those, so there’s some code to do that:

    1. We create new Uint8Array(receivedLength) – a same-typed array with the combined length.
    2. Then use .set(chunk, position) method to copy each chunk one after another in the resulting array.
  5. We have the result in chunksAll. It’s a byte array though, not a string.

    To create a string, we need to interpret these bytes. The built-in TextDecoder does exactly that. Then we can JSON.parse it.

What if we need binary content instead of JSON? That’s even simpler. Instead of steps 4 and 5, we could make a blob of all chunks:

let blob = new Blob(chunks);

Once again, please note, that’s not for upload progress (no way now), only for download progress.

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