Understand asynchronous JavaScript better than 99% of devs by building your own Promise implementation.

• JavaScript core is synchronous.

• When building frontend applications we often need to deal with asynchronous operations, such as user input or HTTP requests.

• How do we deal with this asynchronicity in a synchronous language like JavaScript?

• Callbacks deal asynchronous code and lead to callback hell.

• Promises make callbacks manageable. understand how Promise API work.

• Async/await use the new API to make Promises even easier to work with.

• Synchronicity reduces the cognitive load when writing Javascript code

• A callback is a fucntion that’s passed as an argument to another function to be called by that function at a later time.

• user events are asynchronous.

• Problems with callbacks:

  • Callbacks are the most basic way to deal with asynchronous code.
  • Callbacks create code-flow indirection because the function result is not returned but instead passed into a callback.
  • Handling errors in callbacks is a matter of convention rather than API.
  • Callback hell. “the pyramid of doom”

• Promises

  • is a JS object representing a value that will be available after an asynchronous operation completes.
  • Promise API to access the “promised” value by binding callbacks for different promise states.

• Promise states :

  • pending, set initially
  • fulfilled, successfully
  • rejected, failure

• Work with promises

  • A promise acts as a synchronous value → it can be passed as an argument or returned by functions as usual.
  • To read it’s eventual value you need to “subscribe” to it. To allow that, promise have public methods:
    • promise.then
    • promise.catch
    • promise.finally

• As long as the promise is pending, the callbacks will not be executed.