Fix two js-mode filling bugs

Bug#19399 and Bug#22431:
* lisp/progmodes/js.el (js-mode): Set comment-line-break-function and
c-block-comment-start-regexp.
* test/lisp/progmodes/js-tests.el: New file.

lisp/progmodes/js.el

@@ -3760,6 +3760,8 @@ If one hasn't been set, or if it's stale, prompt for a new one."
         c-line-comment-starter "//"
         c-comment-start-regexp "/[*/]\\|\\s!"
         comment-start-skip "\\(//+\\|/\\*+\\)\\s *")
+  (setq-local comment-line-break-function #'c-indent-new-comment-line)
+  (setq-local c-block-comment-start-regexp "/\\*")
 
   (setq-local electric-indent-chars
 	      (append "{}():;," electric-indent-chars)) ;FIXME: js2-mode adds "[]*".

test/lisp/progmodes/js-tests.el

@@ -0,0 +1,64 @@
+;;; js-tests.el --- Test suite for js-mode
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+(require 'js)
+
+(ert-deftest js-mode-fill-bug-19399 ()
+  (with-temp-buffer
+    (insert "/")
+    (save-excursion (insert "/ comment"))
+    (js-mode)
+    (fill-paragraph)
+    (should (equal (buffer-substring (point-min) (point-max))
+                   "// comment"))))
+
+(ert-deftest js-mode-fill-bug-22431 ()
+  (with-temp-buffer
+    (insert "/**\n")
+    (insert " * Load the inspector's shared head.js for use by tests that ")
+    (insert "need to open the something or other")
+    (js-mode)
+    ;; This fails with auto-fill but not fill-paragraph.
+    (do-auto-fill)
+    (should (equal (buffer-substring (point-min) (point-max))
+                   "/**
+ * Load the inspector's shared head.js for use by tests that need to
+ * open the something or other"))))
+
+(ert-deftest js-mode-fill-bug-22431-fill-paragraph-at-start ()
+  (with-temp-buffer
+    (insert "/**\n")
+    (insert " * Load the inspector's shared head.js for use by tests that ")
+    (insert "need to open the something or other")
+    (js-mode)
+    (goto-char (point-min))
+    (fill-paragraph)
+    (should (equal (buffer-substring (point-min) (point-max))
+                   "/**
+ * Load the inspector's shared head.js for use by tests that need to
+ * open the something or other"))))
+
+(provide 'js-tests)
+
+;;; js-tests.el ends here