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Async Therefore Await

Async Therefore Await

This talk is an introduction to async/await in JavaScript, why it's better than Promises, how it works and how to use it today.

Mohamed Oun

March 15, 2018

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  1. Why even write asynchronous code? • JavaScript is single threaded,

    with lots of processes sharing the same thread • DOM rendering, network requests, your client-side code • They can’t all (and don’t have to) wait for each other
  2. But it’s not so rosy.. • Async code comes with

    added complexity • It’s usually harder to read and reason about • Easy to get into the infamous callback hell with more complex code
  3. How JavaScript handles asynchronous code 1. First, there were callbacks

    2. Then came Promises • .then() and .catch() • Easier to read and understand
  4. Async/Await is ‘syntactic sugar’ over Promises • Use the keyword

    async before your function declaration • Await acts like .then() and pauses the execution until the promise is fulfilled • Looks like synchronous code, but waits in the background, instead of hanging the main thread • We get the performance benefits of asynchronous code and the readability of synchronous code • Best of both worlds!
  5. Because async/await is essentially an abstraction over Promises: • An

    async function is a the same as a function which returns a promise
  6. Because async/await is essentially an abstraction over Promises: • An

    async function that throws an exception is equivalent to a function that returns a rejected Promise
  7. Because async/await is essentially an abstraction over Promises: • You

    can use await Promise.all([]) to run multiple promises in parallel
  8. Top-level await workaround • Top-level (global scope) await doesn’t work

    • You can only await in an async function • But do I call the function I just wrote? • We can use IIFEs to get around that
  9. What if I need to support IE? (Poor you) •

    You can use Babel, specifically babel-preset-env • automatically compiles your code based on the browsers you’re targeting • You can configure it to only include the polyfills and transforms needed for the browsers you support. • Transpiling only what's needed to make your bundles smaller
  10. Resources • Await and Async Explained with Diagrams and Examples

    • async / await in JavaScript - What, Why and How - Fun Fun Function • dotJS 2017 - Wes Bos - Async + Await