Common Causes of TypeError: res.setHeader is not a function in Next.js
- Using the Wrong Object: One of the most common causes of this error is trying to use the `setHeader` method on an object that doesn't have this method. In Next.js, the `res` object typically refers to a Node.js response object when using custom server setups. However, if you are in the context of Next.js API routes or getServerSideProps/getStaticProps functions, the `res` object may be different, or it might not be the object you intend to modify. Misunderstanding which `res` object is being used can lead to this error.
- Client-Side Code Misconception: The `res.setHeader` function is supposed to be used on the server-side. Sometimes, developers mistakenly try to use it in client-side code or in places where a typical Node.js response isn't available. In Next.js, operations that involve `res.setHeader` should be restricted to server-side functions or custom server configurations.
- Using Non-HTTP Libraries: Another scenario is using a non-HTTP library that provides a different kind of `res` object that doesn't support `setHeader`. In such cases, developers might assume they're dealing with a standard HTTP response object while they're not. This is common if mixing libraries that are not meant to work with Node.js's native HTTP server.
- Misconfigured Middleware: Middleware functions that intercept requests may sometimes modify the `res` object or replace it with a custom version. If a middleware changes the response object to a type that doesn't support `setHeader`, subsequent code trying to use this method will throw the error. Depending on how the middleware is configured, it might lead to a different shape of `res` object.
- Improper use of Next.js API Routes: When using Next.js API routes, the handler often has `req` and `res` parameters, but confusion can arise if developers misinterpret what type of response object they have received. For instance, when using API routes, omitting the context of `res` might result in improper assumptions, leading to this error if `res` is attempted to be used as a regular Node.js response.
// Example Code demonstrating possible cause
export default function handler(req, res) {
if (!res.setHeader) {
// Possible custom or improperly configured `res` object
throw new Error("Custom Error Message: res.setHeader is not available");
}
// Setting a header
res.setHeader('Content-Type', 'application/json');
res.status(200).json({ message: 'Hello World' });
}
