handler: add single page application handler

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

handler/spa/spa.go

@@ -0,0 +1,19 @@
+package spa
+
+import (
+	"io/fs"
+	"net/http"
+	"strings"
+)
+
+// Handler serve files from dir and redirect to index if file not exists
+var Handler = func(prefix string, dir fs.FS) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		f := http.StripPrefix(prefix, http.FileServer(http.FS(dir)))
+		if _, err := fs.Stat(dir, strings.TrimPrefix(r.RequestURI, prefix)); err != nil {
+			r.RequestURI = prefix
+			r.URL.Path = prefix
+		}
+		f.ServeHTTP(w, r)
+	}
+}