gob fetcher for wasm example

go/wasmruntime/fetch.go

@@ -5,10 +5,22 @@ package wasmruntime
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"sync"
 	"syscall/js"
 )
 
+// absURL resolves a relative path against the current origin (a browser resolves
+// relative fetch URLs itself, but building the absolute URL keeps behavior
+// uniform — and lets non-browser hosts, e.g. tests, fetch too).
+func absURL(url string) string {
+	if strings.HasPrefix(url, "http://") || strings.HasPrefix(url, "https://") {
+		return url
+	}
+	loc := js.Global().Get("location")
+	return loc.Get("protocol").String() + "//" + loc.Get("host").String() + url
+}
+
 // Fetch performs an HTTP GET via the browser Fetch API and returns the response
 // body as a string. It bridges a JS Promise into Go: it registers then/catch
 // callbacks and blocks the calling goroutine on a channel until the request
@@ -67,7 +79,7 @@ func Fetch(url string) (string, error) {
 		return nil
 	})
 
-	js.Global().Call("fetch", url).Call("then", onResp).Call("catch", onErr)
+	js.Global().Call("fetch", absURL(url)).Call("then", onResp).Call("catch", onErr)
 
 	<-done
 	onResp.Release()
@@ -75,3 +87,63 @@ func Fetch(url string) (string, error) {
 	onErr.Release()
 	return body, ferr
 }
+
+// FetchBytes is like Fetch but returns the raw response body as bytes (via
+// resp.arrayBuffer()), for binary payloads such as gob (Fetch's resp.text()
+// would corrupt non-UTF-8 data). It blocks the calling goroutine the same way —
+// call it from a goroutine and write the result into signals when it returns.
+func FetchBytes(url string) ([]byte, error) {
+	var (
+		once                 sync.Once
+		done                 = make(chan struct{})
+		body                 []byte
+		ferr                 error
+		onResp, onBuf, onErr js.Func
+	)
+	finish := func() { once.Do(func() { close(done) }) }
+
+	onErr = js.FuncOf(func(this js.Value, args []js.Value) any {
+		msg := "fetch error"
+		if len(args) > 0 && args[0].Truthy() {
+			e := args[0]
+			if m := e.Get("message"); m.Truthy() {
+				msg = m.String()
+			} else {
+				msg = e.Call("toString").String()
+			}
+		}
+		ferr = errors.New(msg)
+		finish()
+		return nil
+	})
+
+	onBuf = js.FuncOf(func(this js.Value, args []js.Value) any {
+		if len(args) > 0 {
+			arr := js.Global().Get("Uint8Array").New(args[0])
+			b := make([]byte, arr.Get("length").Int())
+			js.CopyBytesToGo(b, arr)
+			body = b
+		}
+		finish()
+		return nil
+	})
+
+	onResp = js.FuncOf(func(this js.Value, args []js.Value) any {
+		resp := args[0]
+		if !resp.Get("ok").Bool() {
+			ferr = fmt.Errorf("HTTP %d", resp.Get("status").Int())
+			finish()
+			return nil
+		}
+		resp.Call("arrayBuffer").Call("then", onBuf).Call("catch", onErr)
+		return nil
+	})
+
+	js.Global().Call("fetch", absURL(url)).Call("then", onResp).Call("catch", onErr)
+
+	<-done
+	onResp.Release()
+	onBuf.Release()
+	onErr.Release()
+	return body, ferr
+}