examples: tell browsers to not cache wasm files from the example server

src/examples/wasm/README.md

@@ -1,6 +1,6 @@
 # TinyGo WebAssembly examples
 
-The examples here show two different ways of using WebAssembly with TinyGo;
+The examples here show two different ways of using WebAssembly with TinyGo:
 
 1. Defining and exporting functions via the `//go:export <name>` directive. See
 [the export folder](./export) for an example of this.
@@ -31,23 +31,31 @@ $ make main
 
 ## Running
 
-Start the local webserver:
+Start the local web server:
 
 ```bash
 $ go run main.go
 Serving ./html on http://localhost:8080
 ```
 
-`fmt.Println` prints to the browser console.
+Use your web browser to visit http://localhost:8080.
+
+* The wasm "export" example displays a simple math equation using HTML, with
+the result calculated dynamically using WebAssembly.  Changing any of the
+values on the left hand side triggers the exported wasm `update` function to
+recalculate the result.
+* The wasm "main" example uses `println` to write to your browser JavaScript
+console. You may need to open the browser development tools console to see it.
 
 ## How it works
 
-Execution of the contents require a few JS helper functions which are called
-from WebAssembly. We have defined these in
-[wasm_exec.js](../../../targets/wasm_exec.js). It is based on
-`$GOROOT/misc/wasm/wasm_exec.js` from the standard library, but is slightly
-different. Ensure you are using the same version of `wasm_exec.js` as the
-version of `tinygo` you are using to compile.
+Execution of the contents require a few JavaScript helper functions which are
+called from WebAssembly.
+
+We have defined these in [wasm_exec.js](../../../targets/wasm_exec.js).  It is
+based on `$GOROOT/misc/wasm/wasm_exec.js` from the standard library, but is
+slightly different. Ensure you are using the same version of `wasm_exec.js` as
+the version of `tinygo` you are using to compile.
 
 The general steps required to run the WebAssembly file in the browser includes
 loading it into JavaScript with `WebAssembly.instantiateStreaming`, or
@@ -80,8 +88,9 @@ If you have used explicit exports, you can call them by invoking them under the
 `wasm.exports` namespace. See the [`export`](./export/wasm.js) directory for an
 example of this.
 
-In addition to this piece of JavaScript, it is important that the file is served
-with the correct `Content-Type` header set.
+In addition to the JavaScript, it is important the wasm file is served with the
+[`Content-Type`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type)
+header set to `application/wasm`.  Without it, most browsers won't run it.
 
 ```go
 package main
@@ -98,14 +107,31 @@ func main() {
 	fs := http.FileServer(http.Dir(dir))
 	log.Print("Serving " + dir + " on http://localhost:8080")
 	http.ListenAndServe(":8080", http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("Cache-Control", "max-age=0")
+		resp.Header().Add("Cache-Control", "no-store")
+		resp.Header().Add("Cache-Control", "no-cache")
+		resp.Header().Add("Cache-Control", "must-revalidate")
+		resp.Header().Set("Expires", "Thu, 01 Jan 1970 00:00:00 GMT")
 		if strings.HasSuffix(req.URL.Path, ".wasm") {
 			resp.Header().Set("content-type", "application/wasm")
 		}
 
 		fs.ServeHTTP(resp, req)
-	}))
-}
+	}))}
 ```
 
-This simple server serves anything inside the `./html` directory on port `8080`,
-setting any `*.wasm` files `Content-Type` header appropriately.
+This simple server serves anything inside the `./html` directory on port
+`8080`, setting any `*.wasm` files `Content-Type` header appropriately.
+
+For development purposes (**only!**), it also sets various `Cache-Control` and
+`Expires` headers so your browser doesn't cache the files.  This is useful
+while developing, to ensure your browser displays the newest wasm when you
+recompile.
+
+In a production environment you **probably wouldn't** want to set the `Cache-Control`
+and `Expires` headers like this.  Caching is generally beneficial for end users.
+
+Further information on these two headers can be found here:
+
+* https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+* https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expires

src/examples/wasm/server.go

@@ -12,6 +12,11 @@ func main() {
 	fs := http.FileServer(http.Dir(dir))
 	log.Print("Serving " + dir + " on http://localhost:8080")
 	http.ListenAndServe(":8080", http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
+		resp.Header().Set("Cache-Control", "max-age=0")
+		resp.Header().Add("Cache-Control", "no-store")
+		resp.Header().Add("Cache-Control", "no-cache")
+		resp.Header().Add("Cache-Control", "must-revalidate")
+		resp.Header().Set("Expires", "Thu, 01 Jan 1970 00:00:00 GMT")
 		if strings.HasSuffix(req.URL.Path, ".wasm") {
 			resp.Header().Set("content-type", "application/wasm")
 		}