Jeff Lee

@RebootJeff

get comfy & avoid mistakes

Intro to Promises

Who is Jeff?

Full-Stack JavaScriptivore

@RebootJeff

Review of the Basics

Pop Quiz

• What does `.then()` return?
• Should you always return a promise within a `.then()`?
• Is it always ok to replace callbacks with promises?
• Is a segment of a Promise chain always async?
  • What if the code inside the chain is synchronous?
• How do promises work with async-await?
• What does good Promise-based code look like?

Comman Misteaks

Some Anti-Patterns

• Nesting promises to create pyramid of doom
• Explicitly returning/resolving a value as the last step of a promise chain
• Attaching .catch to every .then
• Wrapping every callback in an anonymous function when point-free code works fine

More Anti-Patterns (as told by Bluebird)

• Explicit constructor
  (Unnecessary promisification)
• Old-school `deferred`s
• Handling errors via `.then()`
  • Ex: `.then(success, fail)`

http://bluebirdjs.com/docs/anti-patterns.html

Sweet

Helpers

Have you heard the word

...about Bluebird?

Helpers

• .all
• .spread
• .tap / .tapCatch
• .finally
• .return
• .mapSeries

• .cancel
  (maybe not so common)
• Promise.try
  (to wrap synchronous code so it can be part of a Promise chain; it  can be caught by `.catch` as well)

Caveat on Helpers

Real-World

Practices

Good (maybe even BEST) Practices

• Using .mapSeries to reduce load on requested resource/server
• Error handling
  • top-level catch-all (w/o this, promises will "swallow" your errors)
  • low-level to catch, log, reformat, then re-throw errors (e.g., proxy an AWS SDK error)