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Yesterday I updated to Node.js 4 and today I noticed problems with two apps, Harp and Ionic. Harp was totally broken but Ionic only had issues with the Gulp script. I tried various things, like uninstalling and reinstalling node-sass and Harp itself, but nothing worked. Finally, I found this bug report:
I can confirm that by following the steps here I was able to get Harp working, and I reached out to the team at Harp and they said a proper fix will be out very soon. Update: Harp fixed it.
As for Ionic, it wasn't an issue with Ionic but with gulp-sass. Luckily there is a document on how to fix that too: Update to the latest Gulp Sass. I can confirm this fixed my issues with Ionic's gulp script.