How to update case comment object

I have created a Visualforce page that looks like Close Case Layout which also shows all the associated cases. I am trying to customise the close case to close all the related cases by getting certain inputs from case owner like status, category and comments also.

I am trying to update the comments input into the case comment using onclick javascript ajax toolkit. But it keeps saying invalid field name.

I tried using Casecomments, Casecomment, Casecomment__r,etc.