If you are using any script file and getting "Uncaught ReferenceError: x is not defined " which means ‘x’ is either a variable or a method which you are trying to use before declaring it using var keyword. This means that there is a non-existent variable referenced somewhere. This variable needs to be declared, or you need to make sure it is available in your current script or scope otherwise , it will endup throwing this ‘x’ is not defined error . This usually indicates that your library is not loaded and JavaScript does not recognize the ‘x’.
To solve this error: Load your library at the beginning of all your scripts.
There can be multiple other reasons for this issue:
Conflict with Other Libraries
Path to your library included is not correct
Llibrary file is corrupted
Working offline (when you use CDN)