JavaScript REST API called from a button(callback)

Hello everyone,

I am creating a button in JavaScript (Client Side Extension). I'd like to get a field value from a JSON file.

Example:

when button clicked (on the Opportunity Detail Page):

call a function:

.getJson Opportunity/child/OpportunityRevenues?fields=TotalShareAmount

and save that value on the Opportunity Detail Page in Description/Comments.

Can anyone tell me how to do that without using web services?

Best, Lukasz