If you are familiar with the  Go Playground, then you know how convenient it is to be able to have a Go scratchpad in the browser. Want to show someone a code snippet? Want to quickly test some syntax? Browser-based code pads a helpful. On that note, I  created a new playground. The cool thing about this new playground that it doesn’t use a remote server to run code, just to compile it. The code runs in your browser using  web assembly (WASM).

How Does It Work?

When a user clicks “run”, the code (as text) is sent back to our servers. The server is written in Go. As such the handler for the API looks something like this:

func compileCodeHandler(w http.ResponseWriter, r *http.Request) {
	defer r.Body.Close()

	// Get code from params
	type parameters struct {
		Code string
	}
	decoder := json.NewDecoder(r.Body)
	params := parameters{}
	err := decoder.Decode(&params)
	if err != nil {
		respondWithError(w, 500, "Couldn't decode parameters")
		return
	}

	// create file system location for compilation path
	usr, err := user.Current()
	if err != nil {
		respondWithError(w, 500, "Couldn't get system user")
		return
	}
	workingDir := filepath.Join(usr.HomeDir, ".wasm", uuid.New().String())
	err = os.MkdirAll(workingDir, os.ModePerm)
	if err != nil {
		respondWithError(w, 500, "Couldn't create directory for compilation")
		return
	}
	defer func() {
		err = os.RemoveAll(workingDir)
		if err != nil {
			respondWithError(w, 500, "Couldn't clean up code from compilation")
			return
		}
	}()
	f, err := os.Create(filepath.Join(workingDir, "main.go"))
	if err != nil {
		respondWithError(w, 500, "Couldn't create code file for compilation")
		return
	}
	defer f.Close()
	dat := []byte(params.Code)
	_, err = f.Write(dat)
	if err != nil {
		respondWithError(w, 500, "Couldn't write code to file for compilation")
		return
	}

	// compile the wasm
	const outputBinary = "main.wasm"
	os.Setenv("GOOS", "js")
	os.Setenv("GOARCH", "wasm")
	cmd := exec.Command("go", "build", "-o", outputBinary)
	cmd.Dir = workingDir
	stderr, err := cmd.StderrPipe()
	if err != nil {
		respondWithError(w, 500, err.Error())
		return
	}
	if err := cmd.Start(); err != nil {
		respondWithError(w, 500, err.Error())
		return
	}
	stdErr, err := ioutil.ReadAll(stderr)
	if err != nil {
		respondWithError(w, 500, err.Error())
		return
	}
	stdErrString := string(stdErr)
	if stdErrString != "" {
		parts := strings.Split(stdErrString, workingDir)
		if len(parts) < 2 {
			respondWithError(w, 500, stdErrString)
			return
		}
		respondWithError(w, 400, parts[1])
		return
	}
	if err := cmd.Wait(); err != nil {
		respondWithError(w, 500, err.Error())
		return
	}

	// write wasm binary to response
	dat, err = ioutil.ReadFile(filepath.Join(workingDir, outputBinary))
	if err != nil {
		respondWithError(w, 500, err.Error())
		return
	}
	w.Write(dat)
}

As you can see, the handler simply takes code as input and responds with a slice of WASM bytes.

#golang #wasm #javascript #webassembly