require-json-content-type.md

@teapot-smashers/fetch/require-json-content-type

Require Content-Type header when sending JSON data with fetch

Rule Details

When sending JSON data using fetch() with JSON.stringify() in the body, you must include the appropriate Content-Type: application/json header. Without this header, servers may not properly parse the request body.

❌ Examples of incorrect code for this rule:

// Missing headers entirely
fetch('/api/data', {
  method: 'POST',
  body: JSON.stringify(data),
});

// Missing Content-Type header
fetch('/api/data', {
  method: 'POST',
  headers: {
    Authorization: 'Bearer token',
  },
  body: JSON.stringify(data),
});

// Wrong Content-Type header
fetch('/api/data', {
  method: 'POST',
  headers: {
    'Content-Type': 'text/plain',
  },
  body: JSON.stringify(data),
});

✅ Examples of correct code for this rule:

// Correct Content-Type header
fetch('/api/data', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify(data),
});

// Not using JSON.stringify (rule doesn't apply)
fetch('/api/data', {
  method: 'POST',
  body: 'plain text data',
});

// No body (rule doesn't apply)
fetch('/api/data', {
  method: 'GET',
});

Why?

Without the proper Content-Type header, servers may:

• Not parse the JSON correctly
• Return 400 Bad Request errors
• Treat the body as plain text instead of JSON

When Not To Use It

This rule assumes you're sending data to servers that require the Content-Type header. If you're working with servers that don't require this header, you can disable this rule.

Options

This rule has no options.