Asynchronous Programming

Example: Loading an Image

• Dynamic image loading:

  function addImage(url) {
    const request = new XMLHttpRequest()
    request.open('GET', url)
    request.responseType = 'blob'
  
    request.addEventListener('load', () => {
      const response = request.response
      const blob = new Blob([response], {type: 'image/png'})
      const img = document.createElement('img')
      img.src = window.URL.createObjectURL(blob)
      document.getElementById('images').appendChild(img)   
    })
    request.send()
  }
  

• XMLHttpRequest doesn't block.
• Asynchronous callback when request complete.
• What if you add multiple images?

  addImage('hanafuda/1-1.png')
  addImage('hanafuda/1-2.png')
  addImage('hanafuda/1-3.png')
  addImage('hanafuda/1-4.png')

• Can come in any order.

Demo

images.html

Asynchronous Programming

• JavaScript is single-threaded.
• Each function call runs to completion.
• That is good—no race conditions.
• Can't block in any function.
• Anything after a blocking activity must be done in callback.
• Doing tasks in order requires callbacks inside callbacks.
  • Each callback may require error handling.
  • Callback hell.

    

Promises

The Promise Concept

• Promise object promises to compute a value eventually.
• Easier to chain completion and error actions than with callbacks.
• Making a promise:

  const myPromise = new Promise((resolve, reject) => {   
    const callback = (args) => {
       if (success) resolve(result)
       else reject(reason)
    }
    invokeTask(callback)
  })
  

Example: Loading an Image

function loadImage(url) {
  return new Promise((resolve, reject) => {
    const callback = () => {
      try {
        const response = request.response
        const blob = new Blob([response], {type: 'image/png'})
        const img = document.createElement('img')
        img.src = window.URL.createObjectURL(blob)
        resolve(img)
      } catch (e) {
        reject(e)
      }
    }

    const request = new XMLHttpRequest()
    request.open('GET', url)
    request.responseType = 'blob'
    request.addEventListener('load', callback)
    request.send()
  })  
}

Example: Producing a Value after a Delay

function produceAfterDelay(what, when) {
  return new Promise((resolve, reject) => {
    const callback = () => resolve(what)
    setTimeout(callback, when)
  })                     
}

Promise Control Flow

• When the constructor is called, it invokes its argument (the “executor function”).
  • The executor function returns, then the constructor returns.
  • The promise is in the pending state.
• When the async task has completed, it will invoke a callback that you provided in the executor function.
• If you call reject in a callback, the promise is rejected.
• If you call resolve with an object other than a promise, then the promise is resolved.
• In either of these two cases, the promise is settled.
• If you call resolve with another promise, then this promise stays pending until the other promise is resolved or rejected.
• Caution: Be sure to always call resolve or reject in your callbacks, or the promise never exits the pending state.
• In particular, make sure your callbacks throw no exceptions.
• If an exception is thrown in the executor function, the promise will be rejected.
  • As with all rejections, this happens some time after the constructor returns.

Obtaining Promise Results

• You cannot obtain the result of a promise synchronously.
• Provide instructions what to do when the promise is settled.
• Use the then method to specify action after the promise is resolved.

• const promise1 = loadImage('hanafuda/1-1.png')
  promise1.then(img => {
    document.getElementById('images').appendChild(img)
  })
  

• const promise2 = produceAfterDelay(42, 1000)
  promise2.then(console.log)
  

• then is the only way to get a result out of a promise.

Working with Promises

Promise Chaining

• promise.then(onResolved) is another promise.
• If onResolved(result) returns a promise, then yields that promise.
• Add another then to harvest its result.

  const promise1 = loadImage('hanafuda/1-1.png')
  promise1.then(img => {
    document.getElementById('images').appendChild(img)
    return loadImage('hanafuda/1-2.png') // Another promise
  }).then(img => {
    document.getElementById('images').appendChild(img)
  })

Demo

images2.html

Demo

function produceAfterDelay(what, when) {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(what), when)
  })                     
}

const promise2 = produceAfterDelay(42, 3000)
promise2.then(console.log)

Cleaning Up Promise Chaining

• Implement addImage to return a promise that loads and adds the image to the right place.
• Addd multiple images:

  addImage('hanafuda/1-1.png').then(
    () => addImage('hanafuda/1-2.png'))
  

• Caution: Not

  addImage('hanafuda/1-1.png').then(
    addImage('hanafuda/1-2.png'))
  

• Make more symmetric:

  Promise.resolve()
    .then(() => addImage('hanafuda/1-1.png'))
    .then(() => addImage('hanafuda/1-2.png'))
  

More Promise Chaining

• Can intermingle synchronous and async tasks:

  Promise.resolve()
    .then(() => loadImage('hanafuda/1-1.png'))
    .then(img => append img)
    .then(() => loadImage('hanafuda/1-2.png'))
    .then(img => append img)
  

• Looping over multiple async tasks:

  let p = Promise.resolve()
  for (let i = 1; i <= n; i++) {
    p = p.then(() => loadImage(`hanafuda/1-${i}.png`))
    .then(img => append img)
  }

Demo

images3.html

Fetch API

fetch('https://aws.random.cat/meow')
  .then(result => result.json()) // Asynchronous
  .then(console.log)
    // An object such as { file: 'https://purr.objects-us-east-1.dream.io/i/pBo0r.jpg'}

Rejection Handling

• Can supply rejection handler with then:

  loadImage(url).then(
    (img) => {
      document.getElementById('images').appendChild(img)
    },
    (reason) => {
      console.log(reason)
      document.getElementById('images').appendChild(brokenImage)
    })

• Better to use catch (see next slide).
• That way, errors in the resolve handler are also caught.

Catch

• promise.catch(onReject) calls onRejected(reason) after the promise is rejected.
• Yields a new promise based on the returned value/thrown exception/promise.
• If the handler returns a value, then the resulting promise is resolved.
  • And you can keep on chaining.
• Chain of promises can have a single rejection handler:

  Promise.resolve()
    .then(() => addImage('hanafuda/1-1.png'))
    .then(() => addImage('hanafuda/1-2.png'))
    .catch(reason => console.log('Reason:', reason)) 
  

Finally

• promise.finally(onFinally) calls onFinally after the promise is resolved or rejected.
• No argument is passed—you don't know how the promise settled.
• Intended for cleanup
• Does not change the result/rejection reason of the promise.

Immediately Settled Promises

• Promise.resolve(value) makes a promise that is immediately resolved with the given value.
• Use to kick off asynchronous tasks:

  Promise.resolve('http://...')
    .then(url => loadImage(url))
    .then(img => append img)
  

• Use Promise.resolve(value) to turn a promise/non-promise value into promise.
• If value is a thenable, its then will be called.
• Promise.reject(error) makes a promise that is immediately rejected for the given reason.
• Use when a promise-producing function fails:

  function loadImage(url) {
    if (url === undefined) return Promise.reject(Error('bad url'))
    else new Promise(...)
  }
  

Executing Multiple Promises

• Promise.all(iterable) yields a promise that is resolved when all promises in the iterable are resolved.
• In that case, the combined promise is resolved with an iterable of all promise results.
• If the iterable contained non-promises, they are included in the result iterable.
• If any of the promises rejects, so does Promise.all.
• Promise.race(iterable) runs the promises in the iterable until one of them settles.
• That promise determines the outcome of the race.
• If the iterable contains non-promises, then the first one will be the result.
• If the iterable is empty, then Promise.race never settles.

Demo all and race with images

images5.html

Async and Await

Async Functions

• await waits for a promise to settle and returns its value.
• But wait...you can't keep waiting in a JavaScript function.
• You can...if it is tagged with async:

  async function putImage(url) {
    const img = await loadImage(url)
    document.getElementById('images').appendChild(img)  
  }
  

• async function is transformed—code after await executes when the promise resolves.

Await

• You can't call await in a function that is called from putImage
  • Unless that is also an async function.
• await without promise is wrapped into an immediately resolved promise:

  await true // Same as Promise.resolve(true)

• await on a thenable yields the result of calling then

Async Syntax

• Functions:

  async function(url) { ... }

• Arrow functions:

  async url => { ... }
  async (url, params) => { ... }

• Object literal methods:

  {
    async loadImage(url) { ... },
    ...
  }
  

• Class methods:

  class ImageLoader {
    async load(url) { ... }
  }
  

• In all cases, result is an AsyncFunction instance, not a Function
  • typeof still reports 'function'

Multiple Awaits

• Multiple await are ok:

  async function putImages(url1, url2) {
    const img1 = await loadImage(url1)
    document.getElementById('images').appendChild(img1)  
    const img2 = await loadImage(url2)
    document.getElementById('images').appendChild(img2)  
  }
  

• Loops are ok too:

  async function putImages(urls) {
    for (url of urls) {
      const img = await loadImage(url)
      document.getElementById('images').appendChild(img)
    }
  }

Concurrent Await

• Successive calls to await are done one after another:

  const img1 = await loadImage(url1) 
  const img2 = await loadImage(url2) // Only starts after the first image was loaded
  

• This is probably not want you want. Instead, use Promise.all:

  const [img1, img2] = await Promise.all([loadImage(url1), loadImage(url2)])
  

• But not

  const [img1, img2] = [await loadImage(url1), await loadImage(url2)]
    // Still sequential
  

Async Return Values

• An async function always returns a promise.

  async function getCatImageURL() {
    const result = await fetch('https://aws.random.cat/meow')
    const randomCat = await result.json()
      // A string such as 'https://purr.objects-us-east-1.dream.io/i/mDh7a.jpg'
    return randomCat.file
  }

• Call to get a promise for a string:

  getCatImageURL().then(console.log)
  

• Can also get result with await (in another async function):

  const url = await getCatImageURL()
  

• An async function doesn't have to call await:

  async function hello(who) {
    return `Hello ${who}!`
  }
  
  hello('World').then(console.log)
  

Exceptions in Async Functions

• Throwing an exception in an async function yields a rejected promise.

  async function getAnimalImageURL(type) {
    if (type === 'cat') return await getProperty('https://aws.random.cat/meow', 'file')
    else if (type === 'dog') return await getProperty('https://dog.ceo/api/breeds/image/random', 'message')
    else throw Error('bad type') // Rejected promise
  }
  

• Conversely, rejected promise from await turns into exception:

  async function getAnimalImage(type) {
    try {
      const url = await getAnimalImageURL(type)
      return loadImage(url)
    } catch {
      return BROKEN_IMAGE
    }
  }
  

Async Iterators and Generators

Async Iterators and Generators

• Async iterator: iterator whose next method yields a promise.
  • For a { value: ..., done: ... } object.
• Async iterable: object with a property Symbol.asyncIterable
• ...that is a function yielding an async iterator.
• Easiest way to produce one: async generator:

  async function* loadImages(urls) {
    for (const url of urls) {
      yield await loadImage(url)
    }
  }
  

The for await of Loop

• Consume async iterable like this:

  for await (img of loadImages(urls)) {
    document.getElementById('images').appendChild(img)  
  }

• for await of works with any iterable of promises.
• Must be inside async function.
• Rejected promises are turned into exceptions.
• Cannot use async iterators in for of loop, spread, destructuring.
• Typical use case: paged data

  async function* getPagedData(url) {
    boolean done = false
    const headers = { ... }
    while (url !== undefined) {
      const response = await fetch(url, headers)
      yield response
      url = nextURL(response) // Extract from response headers or body
    }
  }
  

async range

timedrange.js

End of Lesson 10