Javascript Await

4 Key excerpts on "Javascript Await"

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  eBook - ePub

  Mastering TypeScript

  Build enterprise-ready, modular web applications using TypeScript 4 and modern frameworks, 4th Edition

  • Nathan Rozentals(Author)
  • 2021(Publication Date)
  • Packt Publishing

    (Publisher)

  await keyword means that any code after the await call will be paused until the Promise is resolved.

  Note that in an async function, code that is outside of the try catch block will not be executed until the Promise returns.

  Summary

  In this chapter, we focused on the asynchronous nature of the JavaScript runtime, and what techniques we can use to work with this feature. We started with callbacks, which is a standard way of handling asynchronous calls within JavaScript. We also explored the concept of callback hell, which is where using callbacks can become a bit of a nightmare. We then explored Promises, showing how the simple but strict syntax can be applied to any type of asynchronous processing. Finally, we examined the new async await syntax that can be used to pause execution of a code block until a Promise has completed.

  In the next chapter, we will take a look at decorators, and how we can inject functionality into existing code using a simple decorator convention.

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  eBook - ePub

  The TypeScript Workshop

  A practical guide to confident, effective TypeScript programming

  • Ben Grynhaus, Jordan Hudgens, Rayon Hunte, Matt Morgan, Wekoslav Stefanovski(Authors)
  • 2021(Publication Date)
  • Packt Publishing

    (Publisher)

  await keywords to the ECMAScript specification would allow developers to reduce boilerplate and work with promises. The concept comes from the C# programming language, which in turn borrowed the concept of asynchronous workflows from F#.

  An asynchronous function allows a program to continue normal operation even though that function call has yet to return. The program does not wait for that asynchronous function call to complete until the await keyword is found. More significantly, using await will not block the event loop. Even if we have paused part of a program to await the result of an asynchronous function call, other operations can still complete. The event loop is not blocked. For more on the event loop, return to Chapter 12 , Guide to Promises in TypeScript .

  The great thing about these keywords is that they are immediately compatible with promises. We can await any promise, thereby avoiding having to use the then() API. This capability means that along with the concept of promisification (see Chapter 12 , Guide to Promises in TypeScript ), we can use the latest syntax even when integrating with older libraries or modules. To demonstrate this, let's return to an example from the preceding chapter:

  import { promises } from "fs"; promises.readFile('text.txt').then(file => console.log(file.toString()));

  This example uses the promises API from the fs (filesystem) module from Node.js. The code reads a file from the local filesystem and logs the contents to the console. We can use await syntax with this code:

  import { promises } from "fs"; const text = (await promises.readFile('text.txt')).toString(); console.log(text);

  Note that in order to run this code, you must be able to use top-level await , which, at the time of this writing, requires a bit of extra setup. Refer to the section later in this chapter. The takeaway from this example is that we are still able to use the promises API from the fs module, even if we prefer async /await

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  eBook - ePub

  Advanced JavaScript

  Speed up web development with the powerful features and benefits of JavaScript

  • Zachary Shute(Author)
  • 2019(Publication Date)
  • Packt Publishing

    (Publisher)

  Chapter 2

  Asynchronous JavaScript

  Learning Objectives

  By the end of this chapter, you will be able to:

  • Define asynchronous programming
  • Characterize the JavaScript event loop
  • Utilize callbacks and promises to write asynchronous code
  • Simplify asynchronous code with async/await syntax

  In this chapter, we shall learn Asynchronous JavaScript and its uses.

  Introduction

  In the previous chapter, we covered many of the new and powerful features released in ES6. We discussed the evolution of JavaScript and highlighted the key additions in ES6. We discussed scope rules, variable declaration, arrow functions, template literals, enhanced object properties, destructuring assignment, classes and modules, transpiling, and iterators and generators.

  In this chapter, we will learn what an asynchronous programming language is and how to write and understand asynchronous code. In the first topic, we will define asynchronous programming and show how JavaScript is an asynchronous, event driven programming language. Then, we will outline callbacks and show how to use callbacks to write asynchronous JavaScript. We will then define promises and demonstrate how to use promises to write asynchronous JavaScript. In the final topic, we will present the async/await syntax and simplify our asynchronous code using promises and this syntax.

  Asynchronous Programming

  JavaScript is a single threaded, event driven, asynchronous programming language. What does this mean? This means that JavaScript runs on a single thread and delays/handles certain events or function calls through an event queue. We will break down the basics of how JavaScript does this through the following topic.

  Sync Versus Async

  What does it mean for code to be synchronous or asynchronous? These two buzzwords get thrown around a lot in JavaScript. Synchronous is derived from the Greek root syn , meaning "with", and chronos , which means "time". Synchronous literally means "with time", or rather, code that is coordinated with time. Lines of code are run one at a time and are not started until the previous line has been handled. Asynchronous , or async , is derived from the Greek root async

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  eBook - ePub

  Learn ECMAScript

  • Narayan Prusty, MEHUL MOHAN(Authors)
  • 2018(Publication Date)
  • Packt Publishing

    (Publisher)

  async/await , the future of asynchronous programming.

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  async/await – the future of asynchronous programming

  To be honest, async/await blows away whatever you read previously about promises. But hey! You obviously need to know how promises work in order to know how to work with async/await. async/await are built on top of promises; however, once you get used to them, there is no going back to promises (unless, again, you need to convert a callback type API to async/await (you need to use promises for that).)

  About async/await:

  • It's for asynchronous coding
  • It makes code look extremely similar to synchronous coding and thus makes it extremely powerful and easy on the eyes
  • It is built on top of promises
  • It makes error handling a cake walk. You can finally use try and catch with asynchronous coding!
  • ES8 introduced async/await and, by the time you're reading this, it will have been implemented in all browsers natively (at the time of writing, only IE and Opera don't support async/await)

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  async/await versus promises

  Although async/await are actually promises under the hood, they help a lot by adding a ton of readability to code. On the surface level, I believe a developer should be aware of minute differences in the usage of async/await versus promises. Here's a glimpse of these:

  async/await	promises
  Extremely clean code base	Uglier codebase with nested promises
  Error handling with native try-catch blocks	Separate catch() method for error handling
  Syntactic sugar for promises (built on promises)	Native implementation in standard
  Introduced in ES8	Introduced in ES6

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  The async function and await keyword

  In order to use the await keyword, we need to have an async function. The difference between a function and an async function is that the async function is followed by an async

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