Commits on Aug 13, 2016

1. Catches errors from Promises / Async functions …

   This PR adds support for catching errors from rejected promises (perhaps in the
   form of an async function) that matches the existing try/catch behavior for
   synchronous returning functions.
   
   [Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
   have now been part of the JavaScript specification for over a year and are
   popularly used for modeling asynchronous behavior (such as rendering a webpage).
   They're even mentioned in an express [article about best practices](https://expressjs.com/en/advanced/best-practice-performance.html#use-promises).
   
   However one of the pitfalls of using Promises is being aware of error
   behavior and not forgetting to call `.catch()`. This is a prominent note in that
   same article. However a common behavior is to "chain" Promises and allow for the
   top-most Promise to handle the error. For libraries where error-handling is
   critical, like webservers or test harnesses, being Promise-aware and allowing
   functions to return Promises can help isolate error behavior from the library
   user. Mocha is a [great example of this done well](http://paulsalaets.com/testing-with-promises-in-mocha). Users of Express may inadvertently expect this
   behavior, return a Promise from their routing functions, and then be surprised
   when errors are swallowed during debugging.
   
   Support for Promises will become more critical as [Async functions](https://tc39.github.io/ecmascript-asyncawait/) enters the language, giving first-class
   language syntax for creating and operating on promises. This proposal is
   expected to be ratified soon, is already implemented in Chakra-node, is
   implemented behind a flag in Chrome, is expected to arrive in node v7, and is
   already used by many via transpilers like Babel. This makes proper error
   handling for Promises imminent as this use case is about to become far
   more common.
   
   Example of code where Express error handling will work just fine:
   
   ```js
   app.get('/', (req, res) => {
     throw new Error('Oh crap!')
   })
   ```
   
   And a minor syntax modification leads to swallowed errors:
   
   ```js
   app.get('/', async (req, res) => {
     throw new Error('Oh crap!')
   })
   ```
   
   This PR approaches a solution to the problem as minimally-invasive as possible.
   It does not presume anything about the host environment, and it detects and
   supports the [Promise A](http://wiki.commonjs.org/wiki/Promises/A) subset of the
   Promise specification, which many older Promise libraries only support.

   

   leebyron committed Aug 13, 2016
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