Troubleshoot
Properly troubleshooting
When debugging and troubleshooting, it is advised to change your index.html file to use drupalgap.js and jdrupal.js instead of the *.min.js version of the two. This allows your browser's developer tool's console log output to show you the exact line number(s) causing problems.
Common errors
settings.js file
Double check the sitePath variable in the settings.js file is pointing to your Drupal 8 website.
Be careful not to leave any syntax errors in your settings.js file, as this will most likely cause a WSOD for the app.
Testing connection...
If you get stuck with this message on admin/config/services/drupalgap, open your browser's development tool's Console tab and look for the error(s) being thrown. Chances are, you can find that error listed here or mentioned in one of the issue queues.
Failed to load resource: the server responded with a status of 404 (Not Found)
If this happens on the connect?_format=json call, then you probably forgot to set your sitePath value for jDrupal in the settings.js file.
Failed to load resource: the server responded with a status of 406 (Not Acceptable)
If it happens on /user/1?_format=json, then go to admin/config/services/rest in Drupal, and enable User under the Resource name section. Then enable the GET, json and cookie checkboxes under Methods, Accepted request formats and Authentication providers.
Uncaught (in promise) TypeError: window[] is not a function
This probably means you forgot to include a theme's .js file in the <head> of your index.html file.
Uncaught TypeError: a.replace is not a function at Object.dg.getCamelCase
This probably means you've created a form.id element on your form. The id element is reserved by DrupalGap, and overwriting it may lead to unintended consequences.
GET https://www.example.com/node/123?_format=json 406 (Not Acceptable)
You need to enable the Content Resource at admin/config/services/rest, then enable GET, JSON and cookie.
Uncaught (in promise) TypeError: Cannot read property 'js' of undefined Uncaught (in promise) TypeError: Cannot read property 'css' of undefined
On an _attached variable, you probably declared the library name as a string instead of an array.