What is Content Negotiation, and how do you handle the Accept and Content-Type headers?

Short answer

It lets the client specify a preferred data format (such as JSON or XML) through Accept, while Content-Type declares the format of the data actually being sent.

Explanation

When a client sends Accept: application/json, text/csv;q=0.8, the server negotiates the best supported format using the quality factor (q-factor). In Node.js, checking request headers and responding with the correct Content-Type ensures standards compliance across clients and avoids parsing errors.

Code example

const accept = req.headers["accept"] || "";
if (accept.includes("application/json")) {
  res.setHeader("Content-Type", "application/json");
  res.end(JSON.stringify(data));
}

Common mistakes

• Ignoring the request's Content-Type header and trying to parse the body as JSON when it is plain text or multipart data.

Follow-up questions

• When should you prefer Server-Sent Events (SSE) over WebSockets, and how do you implement SSE in Node.js?

Sources

• MDN — Content negotiation