refactor(export): simplify code

src/services/export/md.ts

@@ -5,35 +5,37 @@ import turndownPluginGfm from "joplin-turndown-plugin-gfm";
 
 let instance: TurndownService | null = null;
 
+const fencedCodeBlockFilter: TurndownService.Rule = {
+  filter: function (node, options) {
+    return (
+      options.codeBlockStyle === 'fenced' &&
+      node.nodeName === 'PRE' &&
+      node.firstChild !== null &&
+      node.firstChild.nodeName === 'CODE'
+    )
+  },
+
+  replacement: function (content, node, options) {
+    if (!node.firstChild || !("getAttribute" in node.firstChild) || typeof node.firstChild.getAttribute !== "function") {
+      return content;
+    }
+
+    const className = node.firstChild.getAttribute('class') || ''
+    const language = rewriteLanguageTag((className.match(/language-(\S+)/) || [null, ''])[1]);
+
+    return (
+      '\n\n' + options.fence + language + '\n' +
+      node.firstChild.textContent +
+      '\n' + options.fence + '\n\n'
+    )
+  }
+};
+
 function toMarkdown(content: string) {
     if (instance === null) {
         instance = new TurndownService({ codeBlockStyle: 'fenced' });
-        instance.addRule('fencedCodeBlock', {
-            filter: function (node, options) {
-              return (
-                options.codeBlockStyle === 'fenced' &&
-                node.nodeName === 'PRE' &&
-                node.firstChild !== null &&
-                node.firstChild.nodeName === 'CODE'
-              )
-            },
-          
-            replacement: function (content, node, options) {
-              if (!node.firstChild || !("getAttribute" in node.firstChild) || typeof node.firstChild.getAttribute !== "function") {
-                return content;
-              }
-
-              var className = node.firstChild.getAttribute('class') || ''
-              var language = (className.match(/language-(\S+)/) || [null, ''])[1];
-              language = rewriteLanguageTag(language);
-          
-              return (
-                '\n\n' + options.fence + language + '\n' +
-                node.firstChild.textContent +
-                '\n' + options.fence + '\n\n'
-              )
-            }
-          })
+        // Filter is heavily based on: https://github.com/mixmark-io/turndown/issues/274#issuecomment-458730974 
+        instance.addRule('fencedCodeBlock', fencedCodeBlockFilter);
         instance.use(turndownPluginGfm.gfm);
     }
 
@@ -41,6 +43,10 @@ function toMarkdown(content: string) {
 }
 
 function rewriteLanguageTag(source: string) {
+    if (!source) {
+      return source;
+    }
+
     if (source === "text-x-trilium-auto") {
         return "";
     }