feat(i18n): added fallback non-wasm locale redirection

Perseus will now work with i18n if JS, Wasm, or both are disabled,
meaning progressive enhancement is now complete.

packages/perseus-actix-web/src/initial_load.rs

@@ -8,7 +8,9 @@ use perseus::{
         error_pages::ErrorPageData,
         i18n::{TranslationsManager, Translator},
         router::{match_route, RouteInfo, RouteVerdict},
-        serve::{get_page_for_template, interpolate_page_data},
+        serve::{
+            get_page_for_template, interpolate_locale_redirection_fallback, interpolate_page_data,
+        },
     },
     stores::{ImmutableStore, MutableStore},
     ErrorPages, SsrNode,
@@ -150,12 +152,16 @@ pub async fn initial_load<M: MutableStore, T: TranslationsManager>(
             http_res.body(final_html)
         }
         // For locale detection, we don't know the user's locale, so there's not much we can do except send down the app shell, which will do the rest and fetch from `.perseus/page/...`
-        RouteVerdict::LocaleDetection(_) => {
+        RouteVerdict::LocaleDetection(path) => {
             // We use a `302 Found` status code to indicate a redirect
             // We 'should' generate a `Location` field for the redirect, but it's not RFC-mandated, so we can use the app shell
-            HttpResponse::Found()
-                .content_type("text/html")
-                .body(html_shell.get_ref())
+            HttpResponse::Found().content_type("text/html").body(
+                interpolate_locale_redirection_fallback(
+                    html_shell.get_ref(),
+                    // We'll redirect the user to the default locale
+                    &format!("{}/{}", opts.locales.default, path),
+                ),
+            )
         }
         RouteVerdict::NotFound => html_err(404, "page not found"),
     }

packages/perseus/src/export.rs

@@ -1,5 +1,7 @@
 use crate::errors::*;
-use crate::html_shell::{interpolate_page_data, prep_html_shell};
+use crate::html_shell::{
+    interpolate_locale_redirection_fallback, interpolate_page_data, prep_html_shell,
+};
 use crate::locales::Locales;
 use crate::serve::get_render_cfg;
 use crate::serve::PageData;
@@ -87,12 +89,16 @@ pub async fn export_app(
         };
         // Create a locale detection file for it if we're using i18n
         // These just send the app shell, which will perform a redirect as necessary
+        // Notably, these also include fallback redirectors if either Wasm or JS is disabled (or both)
         // TODO put everything inside its own folder for initial loads?
         if locales.using_i18n {
             immutable_store
                 .write(
                     &format!("exported/{}.html", &initial_load_path),
-                    &html_shell,
+                    &interpolate_locale_redirection_fallback(
+                        &html_shell,
+                        &format!("{}/{}", locales.default, &path),
+                    ),
                 )
                 .await?;
         }

packages/perseus/src/html_shell.rs

@@ -99,3 +99,46 @@ pub fn interpolate_page_data(html_shell: &str, page_data: &PageData, root_id: &s
         .replace(&html_to_replace_double, &html_replacement)
         .replace(&html_to_replace_single, &html_replacement)
 }
+
+/// Intepolates a fallback for locale redirection pages such that, even if JavaScript is disabled, the user will still be redirected to the default locale.
+/// From there, Perseus' inbuilt progressive enhancement can occur, but without this a user directed to an unlocalized page with JS disabled would see a
+/// blank screen, which is terrible UX. Note that this also includes a fallback for if JS is enable dbut Wasm is disabled.
+pub fn interpolate_locale_redirection_fallback(html_shell: &str, redirect_url: &str) -> String {
+    // This will be used if JavaScript is completely disabled (it's then the site's responsibility to show a further message)
+    let dumb_redirector = format!(
+        r#"<noscript>
+    <meta http-equiv="refresh" content="0; url=/{}" />
+</noscript>"#,
+        redirect_url
+    );
+    // This will be used if JS is enabled, but Wasm is disabled or not supported (it's then the site's responsibility to show a further message)
+    // Wasm support detector courtesy https://stackoverflow.com/a/47880734
+    let js_redirector = format!(
+        r#"<script>
+    function wasmSupported() {{
+        try {{
+            if (typeof WebAssembly === "object"
+                && typeof WebAssembly.instantiate === "function") {{
+                const module = new WebAssembly.Module(Uint8Array.of(0x0, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00));
+                if (module instanceof WebAssembly.Module) {{
+                    return new WebAssembly.Instance(module) instanceof WebAssembly.Instance;
+                }}
+            }}
+        }} catch (e) {{}}
+        return false;
+    }}
+
+    if (!wasmSupported()) {{
+        window.location.replace("{}");
+    }}
+</script>"#,
+        redirect_url
+    );
+
+    let html = html_shell.replace(
+        "</head>",
+        &format!("{}\n{}\n</head>", js_redirector, dumb_redirector),
+    );
+
+    html
+}