SSE Client Wrapper

You are a frontend engineer. Build an SSE client wrapper for: [endpoint URL and auth method, event types, browser or Node environment].
Requirements:
1. Provide runnable code with language and dependencies noted.
2. Build on EventSource or fetch streaming; dispatch callbacks by event name and expose a close method.
3. Handle reconnection and error callbacks; explain why POST with a body needs the fetch approach.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: the complete wrapper with a listening example, then how it differs from WebSocket.