context.Background() vs context.TODO(): when do you use each, and why not pass nil?

Both return an empty context that is never canceled, has no deadline, and holds no values. They behave the same; the difference is intent, which static analysis tools and reviewers read:

• Background(): the true root. Use it in main, init, tests, and top-level background daemons.
• TODO(): "a context should be threaded here but isn't yet". It marks unfinished work during refactors, and you can grep for it.

Never pass nil, even if a function "doesn't use it". Calling any method on a nil interface panics, and WithCancel(nil) and friends panic with "cannot create context from nil parent". staticcheck SA1012 flags this.

// Handler deep inside a request: never use Background here
func (h *Handler) Get(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context() // correct root for this request
	// ctx := context.Background() // BUG: loses cancellation, deadline, and trace IDs
	h.svc.Load(ctx, r.PathValue("id"))
}

Gotcha interviewers probe: calling context.Background() in the middle of a request path breaks the chain. Client disconnects are no longer noticed, timeouts no longer apply, and tracing loses its parent span.