bug#8576: 23.2; , js-mode doesn't support multi-line variable declarations

[Dmitry Gutov <dgutov@yandex.ru>]

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> When the first var is staring with an offset, like inside a nested
> function or something, this is failing. It feels like the second line
> is indented wrt to coloumn 0 and not wrt to the "var" statement.

Reproduced when `indent-tabs-mode' is t.

Here's the updated patch for trunk.

-- Dmitry

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diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index 2e943be..98883a9 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -1680,6 +1680,9 @@ This performs fontification according to `js--class-styles'."
           (js--regexp-opt-symbol '("in" "instanceof")))
   "Regexp matching operators that affect indentation of continued expressions.")
 
+(defconst js--declaration-keyword-re
+  (regexp-opt '("var" "let" "const") 'words)
+  "Regular matching variable declaration keywords.")
 
 (defun js--looking-at-operator-p ()
   "Return non-nil if point is on a JavaScript operator, other than a comma."
@@ -1759,6 +1762,42 @@ nil."
          (list (cons 'c js-comment-lineup-func))))
     (c-get-syntactic-indentation (list (cons symbol anchor)))))
 
+(defun js--multiline-decl-indentation ()
+  "Returns the declaration indentation column if the current line belongs
+to a multiline declaration statement.  All declarations are lined up vertically:
+
+var a = 10,
+    b = 20,
+    c = 30;
+"
+  (let (at-opening-bracket)
+    (save-excursion
+      (back-to-indentation)
+      (when (not (looking-at js--declaration-keyword-re))
+        (when (looking-at js--indent-operator-re)
+          (goto-char (match-end 0)))
+        (while (and (not at-opening-bracket)
+                    (not (bobp))
+                    (let ((pos (point)))
+                      (save-excursion
+                        (js--backward-syntactic-ws)
+                        (or (eq (char-before) ?,)
+                            (and (not (eq (char-before) ?\;))
+                                 (and
+                                  (prog2
+                                      (skip-chars-backward "[[:punct:]]")
+                                      (looking-at js--indent-operator-re)
+                                    (js--backward-syntactic-ws))
+                                  (not (eq (char-before) ?\;))))
+                            (and (>= pos (point-at-bol))
+                                 (<= pos (point-at-eol)))))))
+          (condition-case err
+              (backward-sexp)
+            (scan-error (setq at-opening-bracket t))))
+        (when (looking-at js--declaration-keyword-re)
+          (goto-char (match-end 0))
+          (1+ (current-column)))))))
+
 (defun js--proper-indentation (parse-status)
   "Return the proper indentation for the current line."
   (save-excursion
@@ -1769,6 +1808,7 @@ nil."
           ((js--ctrl-statement-indentation))
           ((eq (char-after) ?#) 0)
           ((save-excursion (js--beginning-of-macro)) 4)
+          ((js--multiline-decl-indentation))
           ((nth 1 parse-status)
 	   ;; A single closing paren/bracket should be indented at the
 	   ;; same level as the opening statement. Same goes for