Promises

• Promises are one approach to limiting callback hell.
• Writing code that sets up a promise can be a bit tricky; but
• Writing code that uses a promise that someone else sets up is almost always much cleaner than callback hell.
• Look at ExpressMVCSqlitePromises
• Let’s start by looking at the “end user’s” view in index.js
  • When looking at index, the differences appear minimal. (The callback is just pushed into a then method).
  • The show method demonstrates how to handle errors. Notice that there is a catch method instead of an if/else.
  • This is a moderate improvement, but not a huge deal. (In my opinion.)
• The big improvement comes when we need to nest/chain promises.
  • Look at dbTest.js
  • We want the results of two independant queries (toys below $10 and toys above $100).
  • Promises give us an easy way to run these in parallel and combine the results.