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Javascript Promises and Async Programming


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Promises in Javascript

  • Async Programming
  • Consuming and creating promises
  • Async Await

What problem they solve?

To avoid undefined responses when dealign with external API content retrieved via AJAX calls, we can use Promises as a technique that allow us to ensure we have the data before we try to use it. Callback pyramid of Doom, many levels of nesting indentation of functions, a.k.a. "Callback hell".

  • The "bad" part is that the code becomes dirty and is error prone.
  • Error handling , the pyramid of doom can make the code even messier, adding more indentation.
  • Hard to read

Promises allow us to execute code without massive nesting callbacks.

Promises definition according to Mozilla MDN: Object that represents the eventual completion (or failure) of an async operation, and its value.

Promises have 3 states:

  1. Pending state -> while the promise is happening is pending, until it is...
  2. Fullfied state -> returns a single value grabbed from for example an API.
  3. Rejected state -> the promise fails so it returns a reason why it was rejected similar to a Catch function in a try catch block.
  • Other terms: Setteld and Resolved, both refer to the same state - either Fullfied or Rejected state, it is no longer pending.

Promises in "real life"

We can get data from an API, to see how this works in real life.

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