I want to know how i can make a content which is under workflow uploaded by 'Add Content' link in a space , waiting for approval to be uneditable so that no one can edit it or delete it till the time it is approved or rejected by assignee.
At the start of your workflow you can lock the content as read only using the locking service.
Unfortunately, this service isn't exposed OOTB in the Javascript API. You'll have to either write a custom jBPM action handler or add it to the Javascript API yourself.