convenience-methods.js

// res.bodyUsed is true immediately after the method is called. That is awesome!

fetch('https://jsonplaceholder.typicode.com/posts/1')
  .then(res => {
    console.log('Immediately when the res is received:', res.bodyUsed);

    const body = res.json()
      .then(stuff => {
        console.log('Ok response from res.json', stuff);
        console.log('Body read as JSON', res.bodyUsed);
      });
    console.log('Immediately after trying to read the body', res.bodyUsed);
  });

low-level.js

// res.bodyUsed is true after the first chunk is read

fetch('https://jsonplaceholder.typicode.com/posts/1')
  .then(res => {
    console.log('Immediately when the res is received:', res.bodyUsed);

    const reader = res.body.getReader();
    console.log('After reader acquired', res.bodyUsed);

    const onReadChunk = chunk => {
      if (chunk.done) {
        console.log('Streaming is complete', res.bodyUsed);
      }

      if (!chunk.done) {
        reader.read().then(onReadChunk)
      }
    }

    console.log('Just before the first read', res.bodyUsed);
    // Do the first read().
    reader.read().then(chunk => {
      console.log('Just after first chunk is read', res.bodyUsed);
      onReadChunk(chunk);
    });

    console.log('Immediately after reader.read is called', res.bodyUsed);
  });