add previews to babycode editor component

js/babycode-editor.js

@@ -59,4 +59,48 @@
     e.preventDefault();
     insertTag("code", true)
   })
+  
+  const previewEndpoint = "/api/babycode-preview";
+  let previousMarkup = "";
+  const previewTab = document.getElementById("tab-preview");
+  previewTab.addEventListener("tab-activated", async () => {
+    const previewContainer = document.getElementById("babycode-preview-container");
+    const previewErrorsContainer = document.getElementById("babycode-preview-errors-container");
+    // previewErrorsContainer.textContent = "";
+    const markup = ta.value.trim();
+    if (markup === "" || markup === previousMarkup) {
+      return;
+    }
+    previousMarkup = markup;
+      const req = await fetch(previewEndpoint, {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+        },
+        body: JSON.stringify({markup: markup})
+      })
+      if (!req.ok) {
+        switch (req.status) {
+          case 429:
+            previewErrorsContainer.textContent = "(Old preview, try again in a few seconds.)"
+            previousMarkup = "";
+            break;
+          case 400:
+            previewErrorsContainer.textContent = "(Request got malformed.)"
+            break;
+          case 401:
+            previewErrorsContainer.textContent = "(You are not logged in.)"
+            break;
+          default:
+            previewErrorsContainer.textContent = "(Error. Check console.)"
+            console.error(req.error);
+            break;
+        }
+        return;
+      }
+      
+      const json_resp = await req.json();
+      previewContainer.innerHTML = json_resp.html;
+      previewErrorsContainer.textContent = "";
+  });
 }