Why this simple web server is called even number times?

I'm trying to learn Go web programming, and here is a simple web server: it prints out the times being called.

package main

import (
  "fmt"
  "net/http"
)

var calls int

// HelloWorld print the times being called.
func HelloWorld(w http.ResponseWriter, r *http.Request){
    calls++
    fmt.Fprintf(w, "You've called me %d times", calls)
}

func main() {
  fmt.Printf("Started server at http://localhost%v.\n", 5000)
  http.HandleFunc("/", HelloWorld)
  http.ListenAndServe(":5000", nil)
}

When I refresh the page, I got:

You've called me 1 times
You've called me 3 times
You've called me 5 times
....

Question: Why it is 1, 3, 5 times, rather than 1,2,3...? What's the order of the function HelloWorld being called?

It is because every incoming request is routed to your HelloWorld() handler function, and the browser makes multiple calls under the hood, specifically to /favicon.ico.

And since your web server does not send back a valid favicon, it will request it again when you refresh the page in the browser.

Try it with Chrome: open the Developer tools (CTRL+SHIFT+I), and choose the "Network" tab. Hit refresh, and you will see 2 new entries:

Name          Status   Type
--------------------------------------------------------
localhost     200      document
favicon.ico   200      text/plain

Since your counter starts with 0 (default value for type int), you increment it once and you send back 1. Then the request for favicon.ico increments it again (2), but the result is not displayed. Then if you refresh, it gets incremented again to 3 and you send that back, etc.

Also note that multiple goroutines can serve requests concurrently, so your solution has a race. You should synchronize access to the calls variable, or use the sync/atomic package to increment the counter safely, for example:

var calls int64

func HelloWorld(w http.ResponseWriter, r *http.Request) {
    count := atomic.AddInt64(&calls, 1)
    fmt.Fprintf(w, "You've called me %d times", count)
}

A simple "fix" to achieve what you want would be to check the request path, and if it is not the root "/", don't increment, e.g.:

func HelloWorld(w http.ResponseWriter, r *http.Request) {
    if r.URL.Path != "/" {
        return
    }
    count := atomic.AddInt64(&calls, 1)
    fmt.Fprintf(w, "You've called me %d times", count)
}

You may also choose to only exclude requests for favicon.ico, e.g.:

if r.URL.Path == "/favicon.ico" {
    return
}