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JavaScript Promises
Review

Awesome job! Promises are a difficult concept even for experienced developers, so pat yourself on the back. You’ve learned a ton about asynchronous JavaScript and promises. Let’s review:

  • Promises are JavaScript objects that represent the eventual result of an asynchronous operation.
  • Promises can be in one of three states: pending, resolved, or rejected.
  • A promise is settled if it is either resolved or rejected.
  • We construct a promise by using the new keyword and passing an executor function to the Promise constructor method.
  • setTimeout() is a Node function which delays the execution of a callback function using the event-loop.
  • We use .then() with a success handler callback containing the logic for what should happen if a promise resolves.
  • We use .catch() with a failure handler callback containing the logic for what should happen if a promise rejects.
  • Promise composition enables us to write complex, asynchronous code that’s still readable. We do this by chaining multiple .then()‘s and .catch()‘s.
  • To use promise composition correctly, we have to remember to return promises constructed within a .then().
  • We should chain multiple promises rather than nesting them.
  • To take advantage of concurrency, we can use Promise.all().
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