What is content negotiation in APIs? | API Interview Questions

What You'll Learn

What content negotiation is
Accept and Content-Type headers
Implementation

What is Content Negotiation?

Content negotiation allows clients and servers to agree on the format of data being exchanged.

Key Headers

Accept (Client → Server)

code.jsJavaScript

Accept: application/json
Accept: application/xml
Accept: text/html, application/json;q=0.9

Content-Type (Both directions)

code.jsJavaScript

Content-Type: application/json
Content-Type: application/xml
Content-Type: multipart/form-data

Quality Values (q)

code.jsJavaScript

Accept: application/json;q=1.0, application/xml;q=0.8, */*;q=0.1

// Server preference order:
// 1. JSON (q=1.0)
// 2. XML (q=0.8)
// 3. Anything else (q=0.1)

Implementation

code.jsJavaScript

app.get('/users', (req, res) => {
  const users = [{ id: 1, name: 'John' }];

  // Check Accept header
  const accept = req.accepts(['json', 'xml', 'html']);

  switch (accept) {
    case 'json':
      res.json(users);
      break;
    case 'xml':
      res.type('xml').send(convertToXML(users));
      break;
    case 'html':
      res.render('users', { users });
      break;
    default:
      res.status(406).send('Not Acceptable');
  }
});