I recently updated to the latest stable version of node (v7.5.0) with the following command:
{{< highlight bash “style=emacs” >}} $ nvm install stable —reinstall-packages-from=node {{< /highlight >}}
and when I ran my ios-triage cli app, I saw several DeprecationWarning messages:
{{< highlight bash “style=emacs” >}} (node:3958) DeprecationWarning: Calling an asynchronous function without callback is deprecated. {{< /highlight >}}
Initially I was pretty excited because I recently taught myself node and as a noob I knew I was missing some callbacks which was preventing my async.parallel final function to not run. But the warning wasn’t sufficiently verbose as I needed to know where I was missing the callback.
Tracking down the issue
After a quick google, I discovered you can actually call node with the --trace-deprecation flag which will supply the stacktrace enabling easier debugging. While this is well documented, I didn’t even know what to search for however Google quickly landed me on this stackoverflow question. Below you can see how that helped me quickly pinpoint the cause of error:
{{< highlight bash “style=emacs” >}} $ node —trace-deprecation index.js 9:40:20 AM - info: processed app data 9:40:20 AM - info: backup files processed and written to disk (node:3983) DeprecationWarning: Calling an asynchronous function without callback is deprecated. at maybeCallback (fs.js:96:42) at Object.fs.writeFile (fs.js:1192:14) at /Users/hiro/git/ios-triage/index.js:623:8 at /Users/hiro/git/ios-triage/node_modules/async/dist/async.js:3694:9 at /Users/hiro/git/ios-triage/node_modules/async/dist/async.js:356:16 at replenish (/Users/hiro/git/ios-triage/node_modules/async/dist/async.js:877:25) at iterateeCallback (/Users/hiro/git/ios-triage/node_modules/async/dist/async.js:867:17) at /Users/hiro/git/ios-triage/node_modules/async/dist/async.js:840:16 at /Users/hiro/git/ios-triage/node_modules/async/dist/async.js:3691:13 at apply (/Users/hiro/git/ios-triage/node_modules/async/dist/async.js:21:25) {{< /highlight >}}
Error code and fix
m there, things were pretty simple. In the first few days of getting my mind wrapped around async programming (my first language besides Basic was Pascal and then primarily python), I had even noted in my code that I might need to handle the callback since I wasn’t calling fs.writeFileSync.
This is my faulty code:
{{< highlight javascript “style=emacs” >}} // FIXME should catch errors, maybe use callbacks? fs.writeFile(path.join(processedPath,‘pprofiles.json’), pprofilesJSON, ‘utf8’); callback(null,“processed pprofiles”); {{< /highlight >}}
and here’s the simple fix which captures and handles the callback from writeFile (note: since I’m using the excellent async library I choose not to callback with the error but simply note and handle it in my code):
{{< highlight javascript “style=emacs” >}} // fixed fs.writeFile(path.join(processedPath,‘pprofiles.json’), pprofilesJSON, ‘utf8’, function (err) { if (err) { logger.debug(“error writing pprofile data to disk with error: %s”, err); callback(null, “error writing pprofile data to disk”); } else { callback(null, “wrote pprofile data to disk”); } }); {{< /highlight >}}