Golang : Inject/embed Javascript before sending out to browser example

Problem:

You want to grab some Javascript code from other sources such as an external server or you simply want to separate the Javascript code from your Golang code instead of lumping them together.

Once you grabbed the Javascript code, you want to inject/embed in your HTML code before sending it out to the ResponseWriter/client/browser.

How to do that?

Solution:

Use http.Get() to get JavaScript from another server/URL or use ioutil.ReadFile() to get from file. Next, combine the Javascript code into HTML before writing to ResponseWriter.

Below is an example that you can test out on localhost.

Here you go!

 package main

 import (
 "fmt"
 "io/ioutil"
 "net/http"
 "os"
 )

 func SayHelloWorld(w http.ResponseWriter, r *http.Request) {
 html := `<!DOCTYPE HTML><html><body>Hello World
 `

 response, err := http.Get("http://localhost:8080/js") // or some external sources

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

 defer response.Body.Close()

 javascript, err := ioutil.ReadAll(response.Body) //<--- here!

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

 html = html + string(javascript)
 html = html + `
 </body></html>`

 w.Write([]byte(html))
 }

 func InjectJavaScript(w http.ResponseWriter, r *http.Request) {
 js := `<script type="text/javascript"  charset="utf-8">
 alert("View page source to see injected/embedded Javascript code!");
 </script>`

 w.Write([]byte(js))
 }

 func main() {
 mux := http.NewServeMux()
 mux.HandleFunc("/", SayHelloWorld)
 mux.HandleFunc("/js", InjectJavaScript)
 http.ListenAndServe(":8080", mux)
 }

Hope this helps!

References:

https://www.socketloop.com/tutorials/golang-convert-http-response-body-to-string

https://www.socketloop.com/tutorials/golang-read-file-with-ioutil

https://www.socketloop.com/tutorials/golang-http-server-example

Golang : Inject/embed Javascript before sending out to browser example

See also : Golang : Daemonizing a simple web server process example

By Adam Ng

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