Is there any way to get tabs to align relative to the previous line?

Question

Currently I have this javascript code in a js2-mode buffer after I press tab:

const entries = state.get('entries')
          .concat(getWinners(state.get('vote')));

and I would like for it to look like this aligned according to the dots

const entries = state.get('entries')
                     .concat(getWinners(state.get('vote')));

Is there any way to get emacs to behave like this? indent-relative does not seem to be the answer.

Answer 1

If you don't mind some ugly code, I whipped up something that seems to work:
_{(Disclaimer: I don't know Javascript, but it works on what I've tested)}

(defun my/js2-indent ()
  "If a period is found on the previous line and it's not in a comment
or a string, then indent the current line to the last period on that
line, else indent normally."
  (interactive)
  ;; find column of . if any
  (let ((col (catch 'found
               (save-excursion
                 (back-to-indentation)
                 ;; if the start of the line is not a period (46)
                 ;; or is in a comment then indent normally
                 (when (or (not (eq (char-after) 46))
                           (nth 4 (syntax-ppss)))
                   (throw 'found nil))
                 (end-of-line 0)
                 (backward-char)
                 ;; if the previous line ends with a comment
                 ;; then skip the comment
                 (if (nth 4 (syntax-ppss (1- (point))))
                     (progn
                       (forward-line)
                       (forward-comment -1))
                   ;; syntax-ppss moved point backward
                   ;; so go forward a char
                   (forward-char))
                 ;; while the char isn't a newline (10)
                 (while (not (eq (char-after) 10))
                   ;; break if it's a period (46)
                   ;; and not in a string
                   (if (and (eq (char-after) 46)
                            (not (nth 3 (syntax-ppss))))
                       (throw 'found (current-column))
                     (backward-char)))
                 nil))))
    ;; indent to the found column, otherwise indent normally
    (if col
        (indent-line-to col)
      (js2-indent-line))))

Then to set the indenting so that indent-region or electric-indent-mode don't undo the changes, set the variable indent-line-function to it:

(add-hook 'js2-mode-hook (lambda ()
                           (setq indent-line-function #'my/js2-indent)))