JavaScript — from callbacks to async/await

JavaScript is synchronous. This means that it will execute your code block by order after hoisting. Before the code executes, var and function declarations are “hoisted” to the top of their scope.

This is an example of a synchronous code:

console.log('1')

console.log('2')

console.log('3')

This code will reliably log “1 2 3".

Asynchronous requests will wait for a timer to finish or a request to respond while the rest of the code continues to execute. Then when the time is right a callback will spring these asynchronous requests into action.

This is an example of an asynchronous code:

console.log('1')

setTimeout(function afterTwoSeconds() {
  console.log('2')
}, 2000)

console.log('3')

This will actually log “1 3 2”, since the “2” is on a setTimeout which will only execute, by this example, after two seconds. Your application does not hang waiting for the two seconds to finish. Instead it keeps executing the rest of the code and when the timeout is finished it returns to afterTwoSeconds.

You may ask “Why is this useful?” or “How do I get my async code to become sync?”. Hopefully I can show you the answers.

“The problem”

Let us say our goal is to search for a GitHub user and get all the repositories of that user. The thing is we don’t know the exact name of the user. So we have to list all the users with similar name and their respective repositories.

Doesn’t need to super fancy, something like this

In these examples the request code will use XHR (XMLHttpRequest). You can replace it with jQuery $.ajax or the more recent native approach called fetch. Both will give you the promises approach out of the gate.

It will be slightly changed depending on your approach but as a starter:

// url argument can be something like 'https://api.github.com/users/daspinola/repos'

function request(url) {
  const xhr = new XMLHttpRequest();
  xhr.timeout = 2000;
  xhr.onreadystatechange = function(e) {
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
       // Code here for the server answer when successful
      } else {
       // Code here for the server answer when not successful
      }
    }
  }
  xhr.ontimeout = function () {
    // Well, it took to long do some code here to handle that
  }
  xhr.open('get', url, true)
  xhr.send();
}

Remember that in these examples the important part is not what the end result of the code is. Instead your goal should be to understand the differences of the approaches and how you can leverage them for your development.

Callback

You can save a reference of a function in a variable when using JavaScript. Then you can use them as arguments of another function to execute later. This is our “callback”.

One example would be:

// Execute the function "doThis" with another function as parameter, in this case "andThenThis". doThis will execute whatever code it has and when it finishes it should have "andThenThis" being executed.

doThis(andThenThis)

// Inside of "doThis" it's referenced as "callback" which is just a variable that is holding the reference to this function

function andThenThis() {
  console.log('and then this')
}

// You can name it whatever you want, "callback" is common approach

function doThis(callback) {
  console.log('this first')
  
  // the '()' is when you are telling your code to execute the function reference else it will just log the reference

  callback()
}

Using the callback to solve our problem allows us to do something like this to the request function we defined earlier:

function request(url, callback) {
  const xhr = new XMLHttpRequest();
  xhr.timeout = 2000;
  xhr.onreadystatechange = function(e) {
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
       callback(null, xhr.response)
      } else {
       callback(xhr.status, null)
      }
    }
  }
  xhr.ontimeout = function () {
   console.log('Timeout')
  }
  xhr.open('get', url, true)
  xhr.send();
}

Our function for the request will now accept a callback so that when a request is made it will be called in case of error and in case of success.

const userGet = `https://api.github.com/search/users?page=1&q=daspinola&type=Users`

request(userGet, function handleUsersList(error, users) {
  if (error) throw error
  const list = JSON.parse(users).items

  list.forEach(function(user) {
    request(user.repos_url, function handleReposList(err, repos) {
      if (err) throw err
      // Handle the repositories list here
    })
  })
})

Breaking this down:

• We make a request to get a user’s repositories
• After the request is complete we use callback handleUsersList
• If there is no error then we parse our server response into an object using JSON.parse
• Then we iterate our user list since it can have more than one
• For each user we request their repositories list.
• We will use the url that returned per user in our first response
• We call repos_urlas the url for our next requests or from the first response
• When the request has completed the callback, we will call
• This will handle either its error or the response with the list of repositories for that user

Note: Sending the error first as parameter is a common practice especially when using Node.js.

A more “complete” and readable approach would be to have some error handling. We would keep the callback separate from the request execution.

Something like this:

try {
  request(userGet, handleUsersList)
} catch (e) {
  console.error('Request boom! ', e)
}

function handleUsersList(error, users) {
  if (error) throw error
  const list = JSON.parse(users).items

  list.forEach(function(user) {
    request(user.repos_url, handleReposList)
  })
}

function handleReposList(err, repos) {
  if (err) throw err
  
  // Handle the repositories list here
  console.log('My very few repos', repos)
}

This ends up having problems like racing and error handling issues. Racing happens when you don’t control which user you will get first. We are requesting the information for all of them in case there is more than one. We are not taking an order into account. For example, user 10 can come first and user 2 last. We have a possible solution later in the article.

The main problem with callbacks is that maintenance and readability can become a pain. It sort of already is and the code does hardly anything. This is known as callback hell which can be avoided with our next approach.

Promises

Promises you can make your code more readable. A new developer can come to the code base and see a clear order of execution to your code.

To create a promise you can use:

const myPromise = new Promise(function(resolve, reject) {
  
  // code here
  
  if (codeIsFine) {
    resolve('fine')
  } else {
    reject('error')
  }

})

myPromise
  .then(function whenOk(response) {
    console.log(response)
    return response
  })
  .catch(function notOk(err) {
    console.error(err)
  })

Let us decompose it:

• A promise is initialized with a function that has resolve and reject statements
• Make your async code inside the Promise function
• resolve when everything happens as desired
• Otherwise reject
• When a resolve is found the.then method will execute for that Promise When a reject is found the .catch will be triggered

Things to bear in mind:

• resolve and reject only accept one parameter
• resolve(‘yey’, ‘works’) will only send ‘yey’ to the .then callback function
• If you chain multiple .then Add a return if you want the next .then value not to be undefined
• When a reject is caught with .catch if you have a .then chained to it
• It will still execute that .then You can see the .then as an “always executes” and you can check an example in this comment
• With a chain on .then if an error happens on the first one
• It will skip subsequent .then until it finds a .catch
• A promise has three states
• pending
• When waiting for a resolve or reject to happen
• resolved
• rejected
• Once it’s in a resolved or rejected state
• It cannot be changed

Note: You can create promises without the function at the moment of declarations. The way that I’m showing it is only a common way of doing it.

“Theory, theory, theory…I’m confused” you may say.

Let’s use our request example with a promise to try to clear things up:

function request(url) {
  return new Promise(function (resolve, reject) {
    const xhr = new XMLHttpRequest();
    xhr.timeout = 2000;
    xhr.onreadystatechange = function(e) {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          resolve(xhr.response)
        } else {
          reject(xhr.status)
        }
      }
    }
    xhr.ontimeout = function () {
      reject('timeout')
    }
    xhr.open('get', url, true)
    xhr.send();
  })
}

In this scenario when you execute request it will return something like this:

A promise pending to be resolved or rejected

const userGet = `https://api.github.com/search/users?page=1&q=daspinola&type=Users`

const myPromise = request(userGet)

console.log('will be pending when logged', myPromise)

myPromise
  .then(function handleUsersList(users) {
    console.log('when resolve is found it comes here with the response, in this case users ', users)

    const list = JSON.parse(users).items
    return Promise.all(list.map(function(user) {
      return request(user.repos_url)
    }))
  })
  .then(function handleReposList(repos) {
    console.log('All users repos in an array', repos)
  })
  .catch(function handleErrors(error) {
    console.log('when a reject is executed it will come here ignoring the then statement ', error)
  })

This is how we solve racing and some of the error handling problems. The code is still a bit convoluted. But its a way to show you that this approach can also create readability problems.

A quick fix would be to separate the callbacks like so:

const userGet = `https://api.github.com/search/users?page=1&q=daspinola&type=Users`

const userRequest = request(userGet)

// Just by reading this part out loud you have a good idea of what the code does
userRequest
  .then(handleUsersList)
  .then(repoRequest)
  .then(handleReposList)
  .catch(handleErrors)

function handleUsersList(users) {
  return JSON.parse(users).items
}

function repoRequest(users) {
  return Promise.all(users.map(function(user) {
    return request(user.repos_url)
  }))
}

function handleReposList(repos) {
  console.log('All users repos in an array', repos)
}

function handleErrors(error) {
  console.error('Something went wrong ', error)
}

By looking at what userRequest is waiting in order with the .then you can get a sense of what we expect of this code block. Everything is more or less separated by responsibility.

This is “scratching the surface” of what Promises are. To have a great insight on how they work I cannot recommend enough this article.

Generators

Another approach is to use the generators. This is a bit more advance so if you are starting out feel free to jump to the next topic.

One use for generators is that they allow you to have async code looking like sync.

They are represented by a * in a function and look something like:

function* foo() {
  yield 1
  const args = yield 2
  console.log(args)
}
var fooIterator = foo()

console.log(fooIterator.next().value) // will log 1
console.log(fooIterator.next().value) // will log 2

fooIterator.next('aParam') // will log the console.log inside the generator 'aParam'

Instead of returning with a return, generators have a yield statement. It stops the function execution until a .next is made for that function iteration. It is similar to .then promise that only executes when resolved comes back.

Our request function would look like this:

function request(url) {
  return function(callback) {
    const xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function(e) {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          callback(null, xhr.response)
        } else {
          callback(xhr.status, null)
        }
      }
    }
    xhr.ontimeout = function () {
      console.log('timeout')
    }
    xhr.open('get', url, true)
    xhr.send()
  }
}

We want to have the url as an argument. But instead of executing the request out of the gate we want it only when we have a callback to handle the response.

Our generator would be something like:

function* list() {
  const userGet = `https://api.github.com/search/users?page=1&q=daspinola&type=Users`
 
  const users = yield request(userGet)
  
  yield
  
  for (let i = 0; i<=users.length; i++) {
    yield request(users[i].repos_url)
  }
}

It will:

• Wait until the first request is prepared
• Return a function reference expecting a callback for the first request Our request function accepts a url and returns a function that expects a callback
• Expect a users to be sent in the next .next
• Iterate over users
• Wait for a .next for each of the users
• Return their respective callback function

So an execution of this would be:

try {
  const iterator = list()
  iterator.next().value(function handleUsersList(err, users) {
    if (err) throw err
    const list = JSON.parse(users).items
    
    // send the list of users for the iterator
    iterator.next(list)
    
    list.forEach(function(user) {
      iterator.next().value(function userRepos(error, repos) {
        if (error) throw repos

        // Handle each individual user repo here
        console.log(user, JSON.parse(repos))
      })
    })
  })  
} catch (e) {
  console.error(e)
}

We could separate the callback functions like we did previously. You get the deal by now, a takeaway is that we now can handle each individual user repository list individually.

I have mixed felling about generators. On one hand I can get a grasp of what is expected of the code by looking at the generator.

But its execution ends up having similar problems to the callback hell.

Like async/await, a compiler is recommended. This is because it isn’t supported in older browser versions.

Also it isn’t that common in my experience. So it may generate confusing in codebases maintained by various developers.

An awesome insight of how generators work can be found in this article. And here is another great resource.

Async/Await

This method seems like a mix of generators with promises. You just have to tell your code what functions are to be async. And what part of the code will have to await for that promise to finish.

sumTwentyAfterTwoSeconds(10)
  .then(result => console.log('after 2 seconds', result))

async function sumTwentyAfterTwoSeconds(value) {
  const remainder = afterTwoSeconds(20)
  return value + await remainder
}

function afterTwoSeconds(value) {
  return new Promise(resolve => {
    setTimeout(() => { resolve(value) }, 2000);
  });
}

In this scenario:

• We have sumTwentyAfterTwoSeconds as being an async function
• We tell our code to wait for the resolve or reject for our promise function afterTwoSeconds
• It will only end up in the .then when the await operations finish
• In this case there is only one

Applying this to our request we leave it as a promise as seen earlier:

function request(url) {
  return new Promise(function(resolve, reject) {
    const xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function(e) {
      if (xhr.readyState === 4) {
        if (xhr.status === 200) {
          resolve(xhr.response)
        } else {
          reject(xhr.status)
        }
      }
    }
    xhr.ontimeout = function () {
      reject('timeout')
    }
    xhr.open('get', url, true)
    xhr.send()
  })
}

We create our async function with the needed awaits like so:

async function list() {
  const userGet = `https://api.github.com/search/users?page=1&q=daspinola&type=Users`
  
  const users = await request(userGet)
  const usersList = JSON.parse(users).items
  
  usersList.forEach(async function (user) {
    const repos = await request(user.repos_url)
    
    handleRepoList(user, repos)
  })
}

function handleRepoList(user, repos) {
  const userRepos = JSON.parse(repos)
  
  // Handle each individual user repo here

  console.log(user, userRepos)
}

So now we have an async list function that will handle the requests. Another async is needed in the forEach so that we have the list of repos for each user to manipulate.

We call it as:

list()
  .catch(e => console.error(e))

This and the promises approach are my favorites since the code is easy to read and change. You can read about async/await more in depth here.

A downside of using async/await is that it isn’t supported in the front-end by older browsers or in the back-end. You have to use the Node 8.

You can use a compiler like babel to help solve that.

“Solution”

You can see the end code accomplishing our initial goal using async/await in this snippet.

A good thing to do is to try it yourself in the various forms referenced in this article.

Conclusion

Depending on the scenario you might find yourself using:

• async/await
• callbacks
• mix

It’s up to you what fits your purposes. And what lets you maintain the code so that it is understandable to others and your future self.

Note: Any of the approaches become slightly less verbose when using the alternatives for requests like $.ajax and fetch.

Let me know what you would do different and different ways you found to make each approach more readable.

#javascript #web-development

What is GEEK

Buddha Community

Javascript : Async Await in Javascript | Javascript Interview questions.

Explained in detail about

• Async
• Await
• Fetching country data using Async & Await
• How Async & Await better than promise

Dont miss to watch the video & ask your questions or doubts in comments

https://youtu.be/pJbsd2vAqdI

#javascript #async #await

How To Use Async/Await in JavaScript

How To Use Async/Await in JavaScript

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The async and await is keywords to perform the promise-based asynchronous operation. In this article, we are going to learn how to use async/await in JavaScript.

How to use Async?

It is mandatory to define the async keyword before any function that turns your normal function to async function. Let’s start to learn it through a very basic example.

Normal Function Example

function sayHello() {
    return 'Hello World!';
}

sayHello(); // Hello World!

Copy

Async Function Example

async function sayHello() {
    return 'Hello World!';
}

sayHello(); // [object Promise] { ... }

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We could explicitly return a promise with Promise.resolve() like:

async function sayHello() {
    return Promise.resolve('Hello World!');
}
async function sayHello() {
    return 'Hello World!';
}

let greeting = sayHello();
greeting.then((value) => console.log(value)); // Hello World!

Copy

or you can try this:

async function sayHello() {
    return 'Hello World!';
}

sayHello().then((value) => console.log(value) ); // Hello World!

#javascript #async #await

Understanding JavaScript: Promises, Async & Await!!

Analogy

We all know the importance of promises in our life. We even have a special day dedicated to it :) But how well do we know the importance of promises in JavaScript? Well if you don’t know it yet, it’s a great time to know it because they are becoming more and more popular. So what are promises? Let’s try to understand it through an analogy.

Suppose you are a top class rapper and you haven’t released an album for a while and fans are asking for it day and night. So what you do is that you “promise” them that whenever it will be out, all of them would be notified. To get this done you give your fans a list. They can fill in their email addresses, so that when the album becomes available, all the subscribers instantly receive it. And even if something goes wrong, say a pandemic, so that you can’t release the album, they will still be notified.

Now everyone is happy: You, because the people don’t crowd you anymore, and fans, because they won’t miss any news on the album.

This is a real-life analogy for things we often have in programming:

1. A “producing code” that does something and may take time. That’s a “rapper”.
2. A “consuming code” that wants the result of the “producing code” once it’s ready. Many functions may need that result. These are the “fans”.
3. A promise is a special JavaScript object that links the “producing code” and the “consuming code” together. In terms of our analogy: this is the “subscription list”. The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes that result available to all of the subscribed code when it’s ready.

JavaScript promises are much more complex than a simple subscription list: they have additional features and limitations. But it’s fine to begin with.

#async #promises #javascript #development #await

Javascript async-await: How to Use ES7 async-await

Javascript async-await feature is used to deal with asynchronous code in javascript. The async-await is a just new technique to write async code in a sync function manner.  You can write it as a simple synchronous function. That makes it very popular and understandable. Node.js is now supporting async-await out of the box.

Async-await is a new way to write asynchronous code. Previous options for asynchronous code are  callbacks and  promises. Async/await built on top of promises. It cannot be used with plain callbacks or node callbacks.

Prerequisites

You can read the following tutorials if you do not know anything about Javascript callbacks and promises.

Promises In ES6

Callback in Javascript

#javascript #node.js #async/await

Intro to Callback in JavaScript

During my journey of learning JavaScript and a few days in with React, I had to come to terms with callback functions. It was impossible to avoid its existence and blindly using it without understanding. To understand callback function we have to know a bit about functions. Here I want to talk about function expression, arrow function, and callback function.

---

What is a Callback function?

According to MDN doc:

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

In JavaScript, functions are objects, which means, like any other objects it can be assigned to a variable and passed as an argument to another function. In a nutshell, a callback function is a function that is provided as a parameter for other methods such as forEach method or addEventListener method and gets invoked at a certain point in time.

Passing functions as arguments

So how do we do this? Let’s see with an example below:

document.addEventListener(‘click’,whoAmI);

//whoAmI Function Declaration(Function Statement)
function whoAmI(event){
  console.log(event.target.tagName)
}

We attached the ‘click’ event listener to document with whoAmI function as a parameter that logs the tag name of the clicked target. Whenever ‘click’ happens whoAmI function will be called with an _event_ argument. We call whoAmI a callback function.

When we call a function by its name followed by ( ), we are telling the function to execute its code. When we name a function or pass a function without the ( ), the function does not execute.** The callback function doesn’t execute right away. Instead, the addEventListener method executes the function when the event occurs.**

One more thing I want to mention is because we used function declaration, we were able to call thewhoAmI function before it was declared. It’s the magic of hoisting in JS. But with function expression, it doesn’t get hoisted. So the order of writing function expressions and using them as callback would be crucial.

#callback #javascript #callback-function #function #javascript-fundamental