fix: always return promise if async (#2728)

* fix: always return promise if async

* docs: await walktokens when async

* docs: walktokens async

Co-authored-by: Steven <steven@ceriously.com>

* return rejected promise on error

---------

Co-authored-by: Steven <steven@ceriously.com>

docs/USING_PRO.md

@@ -262,7 +262,7 @@ smartypants('"this ... string"')
 
 <h2 id="walk-tokens">Walk Tokens : <code>walkTokens</code></h2>
 
-The walkTokens function gets called with every token. Child tokens are called before moving on to sibling tokens. Each token is passed by reference so updates are persisted when passed to the parser. The return value of the function is ignored.
+The walkTokens function gets called with every token. Child tokens are called before moving on to sibling tokens. Each token is passed by reference so updates are persisted when passed to the parser. When [`async`](#async) mode is enabled, the return value is awaited. Otherwise the return value is ignored. 
 
 `marked.use()` can be called multiple times with different `walkTokens` functions. Each function will be called in order, starting with the function that was assigned *last*.
 

src/marked.js

@@ -108,28 +108,38 @@ export function marked(src, opt, callback) {
   function onError(e) {
     e.message += '\nPlease report this to https://github.com/markedjs/marked.';
     if (opt.silent) {
-      return '<p>An error occurred:</p><pre>'
+      const msg = '<p>An error occurred:</p><pre>'
         + escape(e.message + '', true)
         + '</pre>';
+      if (opt.async) {
+        return Promise.resolve(msg);
+      }
+      return msg;
+    }
+    if (opt.async) {
+      return Promise.reject(e);
     }
     throw e;
   }
 
   try {
+    if (opt.async) {
+      let promise = Promise.resolve(Lexer.lex(src, opt));
+      if (opt.walkTokens) {
+        promise = promise.then((tokens) =>
+          Promise.all(marked.walkTokens(tokens, opt.walkTokens)).then(() => tokens)
+        );
+      }
+      return promise.then((tokens) => Parser.parse(tokens, opt)).catch(onError);
+    }
+
     const tokens = Lexer.lex(src, opt);
     if (opt.walkTokens) {
-      if (opt.async) {
-        return Promise.all(marked.walkTokens(tokens, opt.walkTokens))
-          .then(() => {
-            return Parser.parse(tokens, opt);
-          })
-          .catch(onError);
-      }
       marked.walkTokens(tokens, opt.walkTokens);
     }
     return Parser.parse(tokens, opt);
   } catch (e) {
-    onError(e);
+    return onError(e);
   }
 }
 

test/unit/marked-spec.js

@@ -1112,4 +1112,14 @@ br
     const html = await promise;
     expect(html.trim()).toBe('<p><em>text walked</em></p>');
   });
+
+  it('should return promise if async', async() => {
+    marked.use({
+      async: true
+    });
+    const promise = marked('*text*');
+    expect(promise).toBeInstanceOf(Promise);
+    const html = await promise;
+    expect(html.trim()).toBe('<p><em>text</em></p>');
+  });
 });