You don't know Promises - IT Konekt April 2019

Transcript

1. You don't know Promises

2. I am Ivan Jovanovic Senior Software Engineer, Tech Speaker You

   can find me at @ivanjov96 or https://ivanjov.com Hello! IJ Consulting

3. Why I don't know Promises?

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6. I am here today To show you how Promises really

   work!

7. JavaScript is strange because ‒ It was built for 10

   days ‒ It has one thread ‒ It is synchronous and asynchronous ‒ It is everywhere ‒ It is different

8. Only one thread And the event loop

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10. How do we handle async code? Promises Callbacks Async/await

11. Callbacks Where everything began... 1

12. “ A callback is a function that is passed as

    an argument and will be executed after another function has finished executing.
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15. Promises Changed everything 2

16. “ The Promise object represents the eventual completion (or failure)

    of an asynchronous operation, and its resulting value.
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19. How Promises really work? The answer is a Microtask queue!

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21. Microtask queue ‒ Independent queue ‒ Used by: process.nextTick, Promises,

    Object.observe, MutationObserver ‒ Faster than the main queue and may be drained several times in a single event loop tick
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23. Next TICK ONE Next TICK TWO TIMEOUT AFTER-ONE TIMEOUT AFTER-TWO

24. Promise methods ‒ Promise.all(iterable) ‒ Promise.race(iterable) ‒ Promise.resolve(value) ‒ Promise.reject(reason)

25. Promise.all(iterable)

26. Rejected situation

27. Promise.race(iterable)

28. Promisify non-Promise code

29. Native Promises from libraries

30. Async/await Comes to the rescue 3

31. “ Async/Await is a special syntax for working with Promises.

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33. Most common problems with Promises that we see every day!

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34. Nested promises 1

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37. Broken Promise chain 2

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40. Mixing sync and async code 3

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44. Missing catch 4

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48. Forget to return a Promise 5

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51. Promisified synchronous code 6

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53. Mixing Promise and Async/Await 7

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56. Async function that returns Promise 8

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60. Define a callback as an async function 9

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62. Clinic.js https://clinicjs.org/

63. Conclusion ‒ JS is crazy and great at the same

    time! ‒ Async behavior is a pro, not a con. ‒ Be careful when using Promises, it's great and dangerous at the same time!

64. Any questions ? You can find me at ‒ @ivanjov96

    ‒ hi@ivanjov.com Thanks!