Slack: add mrkdwn formatter for proper bold/italic/strikethrough rendering

src/slack/format.ts

@@ -0,0 +1,170 @@
+import MarkdownIt from "markdown-it";
+
+type ListState = {
+  type: "bullet" | "ordered";
+  index: number;
+};
+
+type RenderEnv = {
+  slackListStack?: ListState[];
+  slackLinkStack?: { href: string }[];
+};
+
+const md = new MarkdownIt({
+  html: false,
+  linkify: true,
+  breaks: false,
+  typographer: false,
+});
+
+md.enable("strikethrough");
+
+/**
+ * Escape special characters for Slack mrkdwn format.
+ * Slack requires escaping &, <, > to prevent injection.
+ */
+function escapeSlackMrkdwn(text: string): string {
+  return text
+    .replace(/&/g, "&amp;")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;");
+}
+
+function getListStack(env: RenderEnv): ListState[] {
+  if (!env.slackListStack) env.slackListStack = [];
+  return env.slackListStack;
+}
+
+function getLinkStack(env: RenderEnv): { href: string }[] {
+  if (!env.slackLinkStack) env.slackLinkStack = [];
+  return env.slackLinkStack;
+}
+
+md.renderer.rules.text = (tokens, idx) =>
+  escapeSlackMrkdwn(tokens[idx]?.content ?? "");
+
+md.renderer.rules.softbreak = () => "\n";
+md.renderer.rules.hardbreak = () => "\n";
+
+md.renderer.rules.paragraph_open = () => "";
+md.renderer.rules.paragraph_close = (_tokens, _idx, _opts, env) => {
+  const stack = getListStack(env as RenderEnv);
+  return stack.length ? "" : "\n\n";
+};
+
+md.renderer.rules.heading_open = () => "*";
+md.renderer.rules.heading_close = () => "*\n\n";
+
+md.renderer.rules.blockquote_open = () => "&gt; ";
+md.renderer.rules.blockquote_close = () => "\n";
+
+md.renderer.rules.bullet_list_open = (_tokens, _idx, _opts, env) => {
+  getListStack(env as RenderEnv).push({ type: "bullet", index: 0 });
+  return "";
+};
+md.renderer.rules.bullet_list_close = (_tokens, _idx, _opts, env) => {
+  getListStack(env as RenderEnv).pop();
+  return "";
+};
+md.renderer.rules.ordered_list_open = (tokens, idx, _opts, env) => {
+  const start = Number(tokens[idx]?.attrGet("start") ?? "1");
+  getListStack(env as RenderEnv).push({ type: "ordered", index: start - 1 });
+  return "";
+};
+md.renderer.rules.ordered_list_close = (_tokens, _idx, _opts, env) => {
+  getListStack(env as RenderEnv).pop();
+  return "";
+};
+md.renderer.rules.list_item_open = (_tokens, _idx, _opts, env) => {
+  const stack = getListStack(env as RenderEnv);
+  const top = stack[stack.length - 1];
+  if (!top) return "";
+  top.index += 1;
+  const indent = "  ".repeat(Math.max(0, stack.length - 1));
+  const prefix = top.type === "ordered" ? `${top.index}. ` : "• ";
+  return `${indent}${prefix}`;
+};
+md.renderer.rules.list_item_close = () => "\n";
+
+// Slack mrkdwn uses _text_ for italic (same as markdown)
+md.renderer.rules.em_open = () => "_";
+md.renderer.rules.em_close = () => "_";
+
+// Slack mrkdwn uses *text* for bold (single asterisk, not double)
+md.renderer.rules.strong_open = () => "*";
+md.renderer.rules.strong_close = () => "*";
+
+// Slack mrkdwn uses ~text~ for strikethrough (single tilde)
+md.renderer.rules.s_open = () => "~";
+md.renderer.rules.s_close = () => "~";
+
+md.renderer.rules.code_inline = (tokens, idx) =>
+  `\`${escapeSlackMrkdwn(tokens[idx]?.content ?? "")}\``;
+
+md.renderer.rules.code_block = (tokens, idx) =>
+  `\`\`\`\n${escapeSlackMrkdwn(tokens[idx]?.content ?? "")}\`\`\`\n`;
+
+md.renderer.rules.fence = (tokens, idx) =>
+  `\`\`\`\n${escapeSlackMrkdwn(tokens[idx]?.content ?? "")}\`\`\`\n`;
+
+// Slack links use <url|text> format
+md.renderer.rules.link_open = (tokens, idx, _opts, env) => {
+  const href = tokens[idx]?.attrGet("href") ?? "";
+  const stack = getLinkStack(env as RenderEnv);
+  stack.push({ href });
+  return "";
+};
+md.renderer.rules.link_close = (_tokens, _idx, _opts, env) => {
+  const stack = getLinkStack(env as RenderEnv);
+  const link = stack.pop();
+  if (link?.href) {
+    return ` (${escapeSlackMrkdwn(link.href)})`;
+  }
+  return "";
+};
+
+md.renderer.rules.image = (tokens, idx) => {
+  const alt = tokens[idx]?.content ?? "";
+  return escapeSlackMrkdwn(alt);
+};
+
+md.renderer.rules.html_block = (tokens, idx) =>
+  escapeSlackMrkdwn(tokens[idx]?.content ?? "");
+md.renderer.rules.html_inline = (tokens, idx) =>
+  escapeSlackMrkdwn(tokens[idx]?.content ?? "");
+
+md.renderer.rules.table_open = () => "";
+md.renderer.rules.table_close = () => "";
+md.renderer.rules.thead_open = () => "";
+md.renderer.rules.thead_close = () => "";
+md.renderer.rules.tbody_open = () => "";
+md.renderer.rules.tbody_close = () => "";
+md.renderer.rules.tr_open = () => "";
+md.renderer.rules.tr_close = () => "\n";
+md.renderer.rules.th_open = () => "";
+md.renderer.rules.th_close = () => "\t";
+md.renderer.rules.td_open = () => "";
+md.renderer.rules.td_close = () => "\t";
+
+md.renderer.rules.hr = () => "\n";
+
+/**
+ * Convert standard Markdown to Slack mrkdwn format.
+ *
+ * Slack mrkdwn differences from standard Markdown:
+ * - Bold: *text* (single asterisk, not double)
+ * - Italic: _text_ (same)
+ * - Strikethrough: ~text~ (single tilde)
+ * - Code: `code` (same)
+ * - Links: <url|text> or plain URL
+ * - Escape &, <, > as &amp;, &lt;, &gt;
+ */
+export function markdownToSlackMrkdwn(markdown: string): string {
+  const env: RenderEnv = {};
+  const rendered = md.render(markdown ?? "", env);
+  return rendered
+    .replace(/[ \t]+\n/g, "\n")
+    .replace(/\t+\n/g, "\n")
+    .replace(/\n{3,}/g, "\n\n")
+    .trimEnd();
+}