Golang : How to display image file or expose CSS, JS files from localhost?

Problem :

You execute your Golang program on localhost and want to see images on localhost:8080, but your <img> HTML tag is not able to pickup the image file location. This also applicable to all the file types that you want to expose to the world via your webserver. For example, CSS files or JavaScript JS files.

Solution :

You need to expose the directory which contain the image file, CSS or JS files with http.FileServer(http.Dir("./")) functions.

NOTE : Basically, the Golang's webserver will not be able to find the require files - image, CSS, JS - unless they are accessible by the client. Which usually is the Internet browser.

The following code example is a fragment taken from previous tutorial on how to verify token with Google Authenticator App.

 func Home(w http.ResponseWriter, r *http.Request) {

 //w.Write([]byte(fmt.Sprintf("Generating QR code\n")))

 // generate a random string - preferbly 6 or 8 characters
 randomStr := randStr(6, "alphanum")

 // For Google Authenticator purpose
 // for more details see
 // https://github.com/google/google-authenticator/wiki/Key-Uri-Format
 secret = base32.StdEncoding.EncodeToString([]byte(randomStr))
 //w.Write([]byte(fmt.Sprintf("Secret : %s !\n", secret)))

 // authentication link. Remember to replace SocketLoop with yours.
 // for more details see
 // https://github.com/google/google-authenticator/wiki/Key-Uri-Format
 authLink := "otpauth://totp/SocketLoop?secret=" + secret + "&issuer=SocketLoop"

 // Encode authLink to QR codes
 // qr.H = 65% redundant level
 // see https://godoc.org/code.google.com/p/rsc/qr#Level

 code, err := qr.Encode(authLink, qr.H)

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 imgByte := code.PNG()

 // convert byte to image for saving to file
 img, _, _ := image.Decode(bytes.NewReader(imgByte))

 err = imaging.Save(img, "./QRImgGA.png")

 if err != nil {
 fmt.Println(err)
 os.Exit(1)
 }

 // in real world application, the QRImgGA.png file should
 // be a temporary file with dynamic name.
 // for this tutorial sake, we keep it as static name.

 w.Write([]byte(fmt.Sprintf("<html><body><h1>QR code for : %s</h1><img src='http://localhost:8080/QRImgGA.png'>", authLink)))
 w.Write([]byte(fmt.Sprintf("<form action='http://localhost:8080/verify' method='post'>Token : <input name='token' id='token'><input type='submit' value='Verify Token'></form></body></html>")))
 }

 func main() {
 http.HandleFunc("/", Home)
 http.HandleFunc("/verify", Verify)

 // this is for displaying the QRImgGA.png from the source directory
 http.Handle("/QRImgGA.png", http.FileServer(http.Dir("./"))) //<---------------- here!

 http.ListenAndServe(":8080", nil)
 }

Golang : How to display image file or expose CSS, JS files from localhost?

By Adam Ng

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