getResponseSize.js

function getResponseSize(url) {
  return fetch(url).then(response => {
    const reader = response.body.getReader()
    let total = 0
    return reader.read().then(function processResult(result) {
      if (result.done) return total
      const value = result.value
      total += value.length
      console.log('Received chunk', value)
      return reader.read().then(processResult)
    })
  })
}

async function getResponseSize(url) {
  const response = await fetch(url)
  const reader = response.body.getReader()
  let total = 0
  let result = { value: '', done: false }
  
  do {
    result = await reader.read()
    
    const value = result.value
    total += value.length
    console.log('Received chunk', value)
  } while (!result.done)

  return total
}

async function getResponseSizeAsync(url) {
  const response = await fetch(url)
  const reader = response.body.getReader()
  let total = 0
  
  while (true) {
    const result = await reader.read()
    if (result.done) {
      return total
    }

    const value = result.value
    total += value.length
    console.log('Received chunk', value)
  }
}