How to access to the global variable from an anonymous function in the Go?

I have a code:

var test string
func main() {
	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
        test = "index"
    })
    fmt.Println(test)
	if error := http.ListenAndServe(":9001", nil); error != nil {
		log.Fatal("Error!", error)
	}
}

How to change the value of the test variable in the anonymous function? I will be thankful!

The HTTP handler does change your global variable; however you don't protect access to the global variable and thus have a race condition. That is to say fmt.Println(test) runs before your http.HandleFunc.

I'm assuming this is a toy example: If you want to change your code to wait for the value to be changed / wait for an HTTP hit, before terminating, then you could do this:

var test string
var doneCh = make(chan bool, 1)
func main() {
	http.HandleFunc("/", func(writer http.ResponseWriter, request *http.Request) {
		test = "index"
		select {
		case doneCh <- true:
		default:
		}
	})
	go func() {
		<-doneCh
		fmt.Println(test)
	}()
	if error := http.ListenAndServe(":9001", nil); error != nil {
		log.Fatal("Error!", error)
	}
}

This uses a channel to guard the state of test. In real code it would be more likely to use sync.Once or sync.Mutex. Also, I should mention (and I hope you already realize) mutating global state is always something to be avoided when possible.