Answer:
This feature is not available via the Docs API.
More Information
Fundamentally, the Google Docs API lets you create and modify documents.
From the documentation:
The API allows you to do tasks such as the following:
- Automate processes
- Create documentation in bulk
- Generate invoices or contracts
It is not an API which has mehtods that directly replicate the methods avaiable in the user interface.
Workaround:
The Revisions resource of the Drive API might be an approximate solution for you, as it allows revisions of documents to be accessed and downloaded via export links.
As Google Documents have some level of version control implementation, it might be possible to use this to make document comparisons. Unfortunately however, there is no direct way of comparing changes, and as per a recent Issue Tracker case it seems that due to the zip nature of .docx files, not even exporting and comparing MD5s can be a direct solution for this.
Feature Request:
You can however let Google know that this is a feature that is important for the Docs or Drive API and that you would like to request they implement it. You can use the aforelinked Google's Issue Tracker to report issues and make feature requests for their development services.
The page to file a Feature Request for the Google Docs API is here and the Google Drive API is here
I hope this is helpful to you!
References:
Issue Tracker Links: