Mastering Async/Await in JavaScript

May 1, 20184 likes1,708 views

This document outlines the schedule and content for a workshop on mastering async/await in Node.js. The workshop covers using async/await to handle asynchronous code, error handling, and composing async functions. It includes two exercises - one to gather blog post comments from an API, and another to retry failed requests. The key takeaways are that async functions always return promises, await pauses execution until a promise settles, and returned values/errors are handled via promise resolution/rejection.

Mastering Async/Await
Valeri Karpov
@code_barbarian
github.com/vkarpov15

About Me
● Backend Lead @ Booster, Node.js OSS
● Core contributor for Mongoose, Mocha
● Blogger, thecodebarbarian.com
● Early employee at LevelUp, MongoDB, Booster
● Invented the MEAN stack

Workshop Schedule
● 7:30-7:45 Intro, Return Values
● 7:45-8:15 Exercise 1
● 8:15-8:30 Error Handling
● 8:30-9:00 Exercise 2
● 9:00-9:05 Wrap-up and Takeaways

What Is Async/Await?
● 2 new keywords
● Async functions return a promise
● Await pauses execution of an async function

Callback Hell
● Error handling
● Readability

Await and Assignment
● Promise fulfilled value

Composing Async Functions
● Async functions return a promise

Resolved Value vs Return Value
The value you from an async function is
not the return value!

Comparing Resolved and Returned
is equivalent to in a promise

Exercise 1: Gather Blog Post Comments
● The JSONPlaceholder API has 2 endpoints:
○
○

Exercise 1: Gather Blog Post Comments
● a list of blog posts
● comments for each of those posts
● Find the for the comment that contains
“dolorum soluta”
● http://bit.ly/async-await-exercise-1

Part 2: Error Handling
● on a fulfilled promise returns the
computed value
● on a rejected promise throws an error

Consolidated Error Handling
● 3 different patterns to handle all CB errors

Consolidated Error Handling
● Async function try/catch handles sync errors

Unhandled Errors Become Rejections
● Throwing rejects the async function promise

Rejected Value vs Sync Error
● Rejected value like resolved value for errors

Should You Use Try/Catch?
● works too, often a better choice

Try/Catch vs.
● Try/catch for specific, for general
● Don’t use try/catch to wrap the entire function

Exercise 2: Retrying Failed Requests
● Exercise 1 assumed the API was reliable
● What about if every 2nd request fails?
● Need to wrap to retry 3 times
● http://bit.ly/async-await-exercise-2

Key Takeaways
● Async functions always return a promise
● resolves the returned promise
● rejects the returned promise
● pauses execution until promise settles
● returns the value p is fulfilled with

Further Reading
● http://bit.ly/node-promises-from-scratch
● http://bit.ly/async-await-design-patterns
● http://bit.ly/node-async-await
● The 80/20 Guide to ES2015 Generators

Thanks for Attending!
The Mastering Async/Await Ebook, May 23, 2018
tinyletter.com/mastering-async-await

Valeri Karpov will lead a workshop on async/await that includes two exercises. The schedule includes introductions to key concepts like return values, error handling, and loops/conditionals with async functions. Attendees will practice gathering blog post comments and retrying failed requests. Key takeaways are that async functions return promises, return resolves them while throw rejects them, and await pauses execution until a promise settles.

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Mastering Async/Await in JavaScript

1. Mastering Async/Await Valeri Karpov @code_barbarian github.com/vkarpov15

2. About Me ● Backend Lead @ Booster, Node.js OSS ● Core contributor for Mongoose, Mocha ● Blogger, thecodebarbarian.com ● Early employee at LevelUp, MongoDB, Booster ● Invented the MEAN stack

3. Workshop Schedule ● 7:30-7:45 Intro, Return Values ● 7:45-8:15 Exercise 1 ● 8:15-8:30 Error Handling ● 8:30-9:00 Exercise 2 ● 9:00-9:05 Wrap-up and Takeaways

4. What Is Async/Await? ● 2 new keywords ● Async functions return a promise ● Await pauses execution of an async function

5. Callback Hell ● Error handling ● Readability

6. Async/Await Makes Async Logic Flat

7. Loops, If Statements, Try/Catch Work

8. Can Only Await Within An Async Function

9. Await and Assignment ● Promise fulfilled value

10. Composing Async Functions ● Async functions return a promise

11. Resolved Value vs Return Value The value you from an async function is not the return value!

12. Comparing Resolved and Returned is equivalent to in a promise

13. Exercise 1: Gather Blog Post Comments ● The JSONPlaceholder API has 2 endpoints: ○ ○

14. Exercise 1: Gather Blog Post Comments ● a list of blog posts ● comments for each of those posts ● Find the for the comment that contains “dolorum soluta” ● http://bit.ly/async-await-exercise-1

15. Part 2: Error Handling ● on a fulfilled promise returns the computed value ● on a rejected promise throws an error

16. Consolidated Error Handling ● 3 different patterns to handle all CB errors

17. Consolidated Error Handling ● Async function try/catch handles sync errors

18. Unhandled Errors Become Rejections ● Throwing rejects the async function promise

19. Rejected Value vs Sync Error ● Rejected value like resolved value for errors

20. Await Throws, Not the Function Call

21. Should You Use Try/Catch? ● works too, often a better choice

22. Try/Catch vs. ● Try/catch for specific, for general ● Don’t use try/catch to wrap the entire function

23. Exercise 2: Retrying Failed Requests ● Exercise 1 assumed the API was reliable ● What about if every 2nd request fails? ● Need to wrap to retry 3 times ● http://bit.ly/async-await-exercise-2

24. Key Takeaways ● Async functions always return a promise ● resolves the returned promise ● rejects the returned promise ● pauses execution until promise settles ● returns the value p is fulfilled with

25. Further Reading ● http://bit.ly/node-promises-from-scratch ● http://bit.ly/async-await-design-patterns ● http://bit.ly/node-async-await ● The 80/20 Guide to ES2015 Generators

26. Thanks for Attending! The Mastering Async/Await Ebook, May 23, 2018 tinyletter.com/mastering-async-await

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