kelvink96 / javascript_promises

Udacity JavaScript Promises course

#JavaScript Promises Udacity Course

##Lessons

1. Callback vs Promises

• Promise object is used for deferred and asynchronous computations.
• asynchronous - happens at an unknown or unpredictable time

2. Callbacks vs Thens

• Add callback to a function and run at a later time when conditions are met
• errors - how to handle
• sequence of work - use .then

3. Wrapping (Promise), thening (Promise->value->action), catching (Promise->value->recovery), chaining (Promise->value->promise->)

• https://developers.google.com/web/fundamentals/getting-started/primers/promises
• Four States:
  • Fulfilled - resolved
  • Rejected - it didn't work
  • Pending - still waiting
  • Settled - something happened

4. How and when promises executed - event listener set after even fires, never gets called. First resolve promise, then set action for resolution value - then it will fire. Example: new Promise(function(resolve, reject){ resolve('hi'); //works resolve('bye'); //can't happen again }); - promise can only settle once - try and catch wrappers
5. When to use: ajax requests, web worker (api) requests (work on separate threads and post data to main thread)