Understanding FetchError: invalid json response body in Next.js
- When working with Next.js, developers often make use of the `fetch` API to retrieve data from external sources. FetchError can occur when the response body is not valid JSON, indicating a problem with parsing the expected JSON format.
- This error typically arises during server-side data fetching using methods such as `getServerSideProps`, `getStaticProps`, or API routes where the response needs to be parsed to JSON format within the codebase.
Common Scenarios When FetchError Arises
- The response does not adhere to the JSON format, which can happen if the server returns an unexpected content type, or if there's a network issue leading to partial, corrupted data.
- The response body might contain HTML, XML, plain text, or other non-JSON content either due to a misconfigured API or a fallback status page like a 404 or 500 error page returned as HTML.
- Responses that include syntax errors like missing commas, improper brackets, or unquoted keys can also lead to parsing errors when `response.json()` tries to process them.
Effect on Application Behavior
- If this error is not anticipated and handled properly, it can cause the application to fail in rendering the page or cause unexpected behavior especially if the JSON data is crucial for page components.
- User experience might be disrupted due to missing content, sluggish performance due to repeated failed fetch attempts, or improper error display that might confuse end-users.
Working with FetchResponses in Next.js
- When using the `fetch` API within Next.js, it is beneficial to inspect the `response` object for the content type and status before attempting to parse as JSON. This involves utilizing the `response.headers.get('content-type')` and `response.status` to conditionally handle parsing expectations.
- The `response.ok` property can be used to check whether the response was successful (status in the range 200–299), which can be practical for catching and handling non-JSON responses.
export async function getServerSideProps() {
const res = await fetch('https://example.com/data');
if (!res.ok || !res.headers.get('content-type')?.includes('application/json')) {
return { props: { error: 'Invalid response' } };
}
try {
const data = await res.json();
return { props: { data } };
} catch (parseError) {
return { props: { error: 'Failed to parse JSON' } };
}
}
