Fix async mode return value quirk (#602)

* Add test showing bug

* Fix bug demonstrated by the test

* Add release notes

Author: ExplodingCabbage

release-notes.md

@@ -13,6 +13,7 @@
 - [#601](https://github.com/kpdecker/jsdiff/pull/601) - **`diffJson`'s `stringifyReplacer` option behaves more like `JSON.stringify`'s `replacer` argument now.** In particular:
   * Each key/value pair now gets passed through the replacer once instead of twice
   * The `key` passed to the replacer when the top-level object is passed in as `value` is now `""` (previously, was `undefined`), and the `key` passed with an array element is the array index as a string, like `"0"` or `"1"` (previously was whatever the key for the entire array was). Both the new behaviours match that of `JSON.stringify`.
+- [#602](https://github.com/kpdecker/jsdiff/pull/602) - **diffing functions now consistently return `undefined` when called in async mode** (i.e. with a callback). Previously, there was an odd quirk where they would return `true` if the strings being diffed were equal and `undefined` otherwise.
 
 ## 7.0.0
 

src/diff/base.js

@@ -14,7 +14,7 @@ Diff.prototype = {
       value = self.postProcess(value, options);
       if (callback) {
         setTimeout(function() { callback(value); }, 0);
-        return true;
+        return undefined;
       } else {
         return value;
       }
@@ -106,7 +106,7 @@ Diff.prototype = {
 
         if (basePath.oldPos + 1 >= oldLen && newPos + 1 >= newLen) {
           // If we have hit the end of both strings, then we are done
-          return done(buildValues(self, basePath.lastComponent, newString, oldString, self.useLongestToken));
+          return done(buildValues(self, basePath.lastComponent, newString, oldString, self.useLongestToken)) || true;
         } else {
           bestPath[diagonalPath] = basePath;
           if (basePath.oldPos + 1 >= oldLen) {

test/diff/character.js

@@ -59,5 +59,10 @@ describe('diff/character', function() {
       diffChars('whatever', 'whatever', {ignoreCase: true});
       diffChars('whatever', 'whatever', {ignoreCase: true, callback: () => {}});
     });
+
+    it('should return undefined when called in async mode', function() {
+      expect(diffChars('whatever', 'whatever', {callback: () => {}})).to.be.undefined;
+      expect(diffChars('whatever', 'whatever else', {callback: () => {}})).to.be.undefined;
+    });
   });
 });