Shubham Ankit

Shubham Ankit

1557203207

An introduction to JavaScript’s async and await

Asynchronous JavaScript has never been easy. For a while, we used callbacks. Then, we used promises. And now, we have asynchronous functions.

Asynchronous functions make it easier to write asynchronous JavaScript, but it comes with its own set of gotchas that makes life hard for beginners.

In this 2-part series, I want to share everything you need to know about asynchronous functions.

Asynchronous functions

Asynchronous functions contain the async keyword. You can use it in a normal function declaration:

async function functionName (arguments) {
 // Do something asynchronous
}

You can also use it in an arrow-function.

const functionName = async (arguments) => {
 // Do something asynchronous
}

Asynchronous functions always return promises

It doesn’t matter what you return. The returned value will always be a promise.

const getOne = async _ => { 
 return 1 
}

const promise = getOne()
console.log(promise) // Promise 

Note: You should know what are JavaScript Promises and how to use them before you move on. Otherwise, it’ll start to get confusing. Use this article to help you get familiar with JavaScript promises.

The await keyword

When you call a promise, you handle the next step in a then call, like this:

const getOne = async _ => { 
 return 1 
}

getOne()
 .then(value => {
 console.log(value) // 1
 })

The await keyword lets you wait for the promise to resolve. Once the promise is resolved, it returns the parameter passed into the then call.

const test = async _ => {
 const one = await getOne()
 console.log(one) // 1
}
test()

Return await

There’s no need to await before returning a promise. You can return the promise directly.

(If you return await something, you resolve the original promise first. Then, you create a new promise from the resolved value. return await effectively does nothing. No need for the extra step).

// Don’t need to do this 
const test = async _ => {
 return await getOne()
}
test()
 .then(value => {
 console.log(value) // 1
 })

// Do this instead
const test = async _ => {
 return getOne()
}
test()
 .then(value => {
 console.log(value) // 1
 })

Note: If you don’t need await, you don’t need to use an async function. The example above can be rewritten as follows:

// Do this instead
const test = _ => {
 return getOne()
}
test()
 .then(value => {
 console.log(value) // 1
 })

Handling errors

If a promise results in an error, you handle it with a catch call, like this:

const getOne = async (success = true) => { 
 if (success) return 1
 throw new Error(‘Failure!’)
}
getOne(false)
 .catch(error => console.log(error)) // Failure!

If you want to handle an error in an asynchronous function, you need to use a try/catch call.

const test = async _ => {
 try {
   const one = await getOne(false)
 } catch (error) {
   console.log(error) // Failure!
 }
}
test()

If you have multiple await keywords, error handling can become ugly…

const test = async _ => {
   try {
     const one = await getOne(false)
   } catch (error) {
     console.log(error) // Failure!
   }
  try {
    const two = await getTwo(false)
  } catch (error) {
    console.log(error) // Failure!
  }
  try {
    const three = await getThree(false)
  } catch (error) {
    console.log(error) // Failure!
  }
}
test()

There’s a better way.

We know that asynchronous functions always return a promise. When we call a promise, we can handle errors in a catch call. This means we can handle any errors from our asynchronous function by adding .catch.

const test = async _ => {
 const one = await getOne(false)
 const two = await getTwo(false)
 const three = await getThree(false)
}
test()
 .catch(error => console.log(error)))

Note: The Promise catch method lets you catch one error only.

Multiple awaits

await blocks JavaScript from executing the next line of code until a promise resolves. This may have the unintended consequence of slowing down code execution.

To show this in action, we need to create a delay before resolving the promise. We can create a delay with a sleep function.

const sleep = ms => {
 return new Promise(resolve => setTimeout(resolve, ms))
}

ms is the number of milliseconds to wait before resolving. If you pass in 1000 into sleep, JavaScript will wait for one second before resolving the promise.

// Using Sleep
console.log(‘Now’)
sleep(1000)
 .then(v => { console.log(‘After one second’) })

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Console logs ‘now’ immediately. One second later, it logs ‘After one second’

Let’s say getOne takes one second to resolve. To create this delay, we pass 1000 (one second) into sleep. After one second has passed and the sleep promise resolves, we return the value 1.

const getOne = _ => {
 return sleep(1000).then(v => 1)
}

If you await getOne(), you’ll see that it takes one second before getOne resolves.

const test = async _ => {
 console.log(‘Now’)

 const one = await getOne()
 console.log(one)
}
test()

Image for post

Image for post

Console logs ‘Now’ immediately. After one second, console logs 1

Now let’s say you need to wait for three promises. Each promise has a one-second delay.

const getOne = _ => {
 return sleep(1000).then(v => 1)
}

const getTwo = _ => {
 return sleep(1000).then(v => 2)
}
const getThree = _ => {
 return sleep(1000).then(v => 3)
}

If you await these three promises in a row, you’ll have to wait for three seconds before all three promises get resolved. This is not good because we forced JavaScript to wait two extra seconds before doing what we need to do.

const test = async _ => {
 const one = await getOne()
 console.log(one)

 const two = await getTwo()
 console.log(two)
 const three = await getThree()
 console.log(three)
 console.log(‘Done’)
}
test()

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Console shows ‘Now’ immediately. One second later, it shows 1. Another second later, it shows 2. Another second later, it shows 3 and ‘Done’ at the same time.

If getOnegetTwo and getThree can be fetched simultaneously, you’ll save two seconds. You can fetch these three promises at the same time with Promise.all.

There are three steps:

1. Create the three promises

2. Add all three promises into an array

3. await the array of promises with Promise.all

Here’s what it looks like:

const test = async _ => {
 const promises = [getOne(), getTwo(), getThree()]
 console.log(‘Now’)

 const [one, two, three] = await Promise.all(promises)
 console.log(one)
 console.log(two)
 console.log(three)
 console.log(‘Done’)
}
test()

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Console shows ‘Now’ immediately. After one second, console shows 1, 2, 3, and ‘Done’

That’s all you need to know about basic asynchronous functions! I hope this article clears things up for you.

Note: This article is a modified excerpt from Learn JavaScript. If you find this article useful, you might want to check it out.

Next up, we’re going to look at asynchronous functions and its behavior in loops.

This article was originally posted on blog

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An introduction to JavaScript’s async and await

Javascript : Async Await in Javascript | Javascript Interview questions.

Explained in detail about

  • Async
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  • 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

Introduction With Basic JavaScript

The world’s most misunderstood programming language is JavaScript but JavaScript is now used by an incredible number of high-profile applications. So, it’s an important skill for any web or mobile developer to enrich the deeper knowledge in it.

Unlike most programming languages, the JavaScript language has no concept of input or output. It is designed to run as a scripting language in a host environment, and it is up to the host environment to provide mechanisms for communicating with the outside world.

Its syntax is based on the Java and C languages — many structures from those languages apply to JavaScript as well. JavaScript supports object-oriented programming with object prototypes, instead of classes. JavaScript also supports functional programming — because they are objects, functions may be stored in variables and passed around like any other object.

Let’s start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript’s types are:

· Number

· String

· Boolean

· Function

· Object

· Symbol

and undefined and null, which are … slightly odd. And Array, which is a special kind of object. Date and RegExp, which are objects that you get for free. And to be technically accurate, functions are just a special type of object. So the type of diagram looks like this:

#beginner-javascript #javascript #javascript-introduction #javascript-fundamental #basic-javascritp

Sadie  Cassin

Sadie Cassin

1598637660

How To Use Async/Await in JavaScript

How To Use Async/Await in JavaScript

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Async/Await

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] { ... }

Copy

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

Giles  Goodwin

Giles Goodwin

1600929360

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. “producing code” that does something and may take time. That’s a “rapper”.
  2. “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