In Next.js, you can pass a variable from middleware to an API route by attaching it to the req object, which is available in both the middleware and API route functions. To achieve this, you need to make sure that the middleware runs before the API route is processed.

Here's a step-by-step guide on how to pass a variable from middleware to an API route in Next.js:

  1. Create a Custom Middleware: In the root of your Next.js project, create a new file (e.g., myMiddleware.js) for your custom middleware.

    // myMiddleware.js export default function myMiddleware(req, res, next) { // Modify or add the variable you want to pass to the API route req.customVariable = 'Hello from middleware!'; next(); }
  2. Register the Middleware: In your Next.js configuration (e.g., next.config.js), you can register your custom middleware using the serverMiddleware option. This ensures that the middleware runs on every incoming request.

    // next.config.js module.exports = { serverMiddleware: [ // Add the path to your middleware file { path: '/api', handler: '~/myMiddleware.js' }, ], };
  3. Access the Variable in the API Route: Now, in your API route handler, you can access the variable that was set in the middleware through the req object.

    // pages/api/myapi.js export default function handler(req, res) { const customData = req.customVariable; res.status(200).json({ message: customData }); }

When a request is made to the /api/myapi route, the middleware will run first, setting the customVariable on the req object. Then, the API route will be executed, and it can access the variable through the req object to respond accordingly.

Remember that middleware in Next.js runs on the server-side, so any variable set in the middleware will be available only on the server-side and will not be passed to the client-side or browser. If you need to pass data from the server to the client, you should consider using props or the Next.js data fetching methods on the client side.

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