Streaming Interview Questions

Create an AbortController per request, pass controller.signal to fetch, and call controller.abort() from the Stop button or a useEffect cleanup. The fetch promise or the pending reader.read() then rejects with an AbortError, which you catch and treat as a normal stop, keeping the text already shown. On the server, pass the incoming request signal on to the upstream LLM call so the model actually stops generating and billing.

Start the upstream request, then hand its body stream straight to your response instead of awaiting text() or json(). In a Next.js Route Handler that means returning new Response(upstream.body); in Express, pipe Readable.fromWeb(upstream.body) into res. Then remove hidden buffers along the path (compression middleware, nginx proxy buffering, caches) and abort the upstream call when the client disconnects.

response.body is a ReadableStream of Uint8Array chunks. Get a reader with getReader(), call read() in a loop until done is true, and turn each chunk into text with a single TextDecoder using decode(chunk, { stream: true }). Then buffer the text and parse only complete lines or events, because network chunks do not line up with tokens or messages.

UTF-8 encodes many characters in 2 to 4 bytes, and the network can split a stream anywhere, including in the middle of one character. If you decode each chunk on its own, the incomplete bytes become the replacement character U+FFFD. Use one TextDecoder for the whole stream with decode(chunk, { stream: true }) (or TextDecoderStream), and flush it at the end, so incomplete bytes wait for the next chunk.

For a normal chat completion, Server-Sent Events (SSE) over HTTP is the better fit: the client sends one request, the server pushes tokens back in one direction, and it works with ordinary HTTP infrastructure, auth and serverless functions. The major LLM APIs stream over SSE for exactly this reason. WebSockets make sense when both sides need to keep sending data over one long-lived connection, such as realtime voice or a persistent agent session.