While attempting to upgrade from version 5.2 to 6.0 of Angular, the following error occurred: "Error: An abrupt termination of JSON input was encountered."

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This error is most likely caused by an issue with the JSON files in your Angular project. It could be that the JSON files are malformed or missing some required syntax.

To solve this issue, you should first check all of your JSON files to make sure they are properly formatted with no missing syntax or errors. You can use online JSON validators to check each file for errors.

Additionally, you should check if any of your package.json files are malformed. A missing comma or incorrect version number could be causing this error.

Once you have fixed all of the JSON errors in your project, try upgrading to Angular 6.0 again. Hopefully, this will resolve the "Error: An abrupt termination of JSON input was encountered" error.