Slack: add media block fallback text handling

src/slack/blocks-fallback.ts

@@ -0,0 +1,95 @@
+import type { Block, KnownBlock } from "@slack/web-api";
+
+type PlainTextObject = { text?: string };
+
+type SlackBlockWithFields = {
+  type?: string;
+  text?: PlainTextObject & { type?: string };
+  title?: PlainTextObject;
+  alt_text?: string;
+  elements?: Array<{ text?: string; type?: string }>;
+};
+
+function cleanCandidate(value: string | undefined): string | undefined {
+  if (typeof value !== "string") {
+    return undefined;
+  }
+  const normalized = value.replace(/\s+/g, " ").trim();
+  return normalized.length > 0 ? normalized : undefined;
+}
+
+function readSectionText(block: SlackBlockWithFields): string | undefined {
+  return cleanCandidate(block.text?.text);
+}
+
+function readHeaderText(block: SlackBlockWithFields): string | undefined {
+  return cleanCandidate(block.text?.text);
+}
+
+function readImageText(block: SlackBlockWithFields): string | undefined {
+  return cleanCandidate(block.alt_text) ?? cleanCandidate(block.title?.text);
+}
+
+function readVideoText(block: SlackBlockWithFields): string | undefined {
+  return cleanCandidate(block.title?.text) ?? cleanCandidate(block.alt_text);
+}
+
+function readContextText(block: SlackBlockWithFields): string | undefined {
+  if (!Array.isArray(block.elements)) {
+    return undefined;
+  }
+  const textParts = block.elements
+    .map((element) => cleanCandidate(element.text))
+    .filter((value): value is string => Boolean(value));
+  return textParts.length > 0 ? textParts.join(" ") : undefined;
+}
+
+export function buildSlackBlocksFallbackText(blocks: (Block | KnownBlock)[]): string {
+  for (const raw of blocks) {
+    const block = raw as SlackBlockWithFields;
+    switch (block.type) {
+      case "header": {
+        const text = readHeaderText(block);
+        if (text) {
+          return text;
+        }
+        break;
+      }
+      case "section": {
+        const text = readSectionText(block);
+        if (text) {
+          return text;
+        }
+        break;
+      }
+      case "image": {
+        const text = readImageText(block);
+        if (text) {
+          return text;
+        }
+        return "Shared an image";
+      }
+      case "video": {
+        const text = readVideoText(block);
+        if (text) {
+          return text;
+        }
+        return "Shared a video";
+      }
+      case "file": {
+        return "Shared a file";
+      }
+      case "context": {
+        const text = readContextText(block);
+        if (text) {
+          return text;
+        }
+        break;
+      }
+      default:
+        break;
+    }
+  }
+
+  return "Shared a Block Kit message";
+}