I have an approve reject workflow attached to one of the document library. Is there any way I can "approve" or "reject" the item using a built in websevice, just like we approve/reject through the workflow form?
I just did that using Workflow.asmx's AlterToDo. Basically closing the Task associated with the workflow completes the workflow. I was able to do that using Workflow.asmx but updating Tasks List directly with REST api will also do the same.
As mentioned in this thread, setting "TaskStatus" as "Approved" is mandatory to set the workflow completed.