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Write a Custom Middleware

January ships with built-in middleware package, which is initialised in the init.january.go file.
Adding a middleware to your application is 2 simple steps:
  1. Write the Middleware
  2. Wire it to the January Router
For this tutorial let’s write a simple CORS middle.

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1. Writing a Middleware

Create a file named cors.go in the middleware package and add a method called Cors with a receiver m *Middleware
middleware/cors.go
func (m *Middleware) Cors(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {

	})
}
now, let’s add the middleware logic to our cors middleware:
middleware/cors.go
func (m *Middleware) Cors(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Set CORS headers
		w.Header().Set("Access-Control-Allow-Origin", "*")
		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
		w.Header().Set("Access-Control-Allow-Headers", "Accept, Authorization, Content-Type, X-CSRF-Token, X-Requested-With")
		w.Header().Set("Access-Control-Allow-Credentials", "true")
		w.Header().Set("Access-Control-Max-Age", "86400")

		// Handle preflight requests
		if r.Method == "OPTIONS" {
			w.WriteHeader(http.StatusOK)
			return
		}

		// Call the next handler
		next.ServeHTTP(w, r)
	})
}

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2. Wiring to the January Router

In the init.january.go file, add the middle just below the apply middleware comment:
init.january.go
func initApplication() *application {
	jan.AppName = "github.com/akshanshgusain/my_crud_srv"

	mid := &middleware.Middleware{
		App: jan,
	}
	// ...

	// apply middlewares
	app.App.Routes.Use(mid.Cors)

	// ...

	return app
}

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Try it out

Run the application with make start 
> make start
Building January...
January built!
Starting January...
January started!

 ~/Downloads/my_crud_srv  INFO  2025/05/18 16:41:54 load session called                                                                                         ok | base py
INFO    2025/05/18 16:41:54 Starting January server at http://127.0.0.1:9095/
INFO    2025/05/18 16:41:54 Quit the server with control+c
with your application running, you can test the middleware: 
 curl -X OPTIONS -i http://localhost:9095 -H "Origin: http://example.com" -H "Access-Control-Request-Method: POST"