Use js2-mode to print json path

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CREATED: <2015-10-29 Thu>

UPDATED: <2015-11-22 Sun>

These days most web applications are based on RESTful.

So javascript developers have to deal with JSON frequently.

When working on JSON, the most boring and error prone task is to get the full path of specific field.

For example, given the JSON response like: 

{
  key1: {
    subkey1: {
      arr1: [{ key3: 'hello'}]
    }
  }
}

The developer need figure out the full path of "key3" which is "key1.subkey1.arr1[0].key3".

My solution is to take advantage of AST created by js2-mode.

Step 1, insert below code into ~/.emacs, 

;; {{ print json path, will be removed when latest STABLE js2-mode released
(defun js2-get-element-index-from-array-node (elem array-node &optional hardcoded-array-index)
  "Get index of ELEM from ARRAY-NODE or 0 and return it as string."
  (let ((idx 0) elems (rlt hardcoded-array-index))
    (setq elems (js2-array-node-elems array-node))
    (if (and elem (not hardcoded-array-index))
        (setq rlt (catch 'nth-elt
                    (dolist (x elems)
                      ;; We know the ELEM does belong to ARRAY-NODE,
                      (if (eq elem x) (throw 'nth-elt idx))
                      (setq idx (1+ idx)))
                    0)))
    (format "[%s]" rlt)))

(defun js2-print-json-path (&optional hardcoded-array-index)
  "Print the path to the JSON value under point, and save it in the kill ring.
If HARDCODED-ARRAY-INDEX provided, array index in JSON path is replaced with it."
  (interactive "P")
  (let (previous-node current-node
        key-name
        rlt)

    ;; The `js2-node-at-point' starts scanning from AST root node.
    ;; So there is no way to optimize it.
    (setq current-node (js2-node-at-point))

    (while (not (js2-ast-root-p current-node))
      (cond
       ;; JSON property node
       ((js2-object-prop-node-p current-node)
        (setq key-name (js2-prop-node-name (js2-object-prop-node-left current-node)))
        (if rlt (setq rlt (concat "." key-name rlt))
          (setq rlt (concat "." key-name))))

       ;; Array node
       ((or (js2-array-node-p current-node))
        (setq rlt (concat (js2-get-element-index-from-array-node previous-node
                                                                 current-node
                                                                 hardcoded-array-index)
                          rlt)))

       ;; Other nodes are ignored
       (t))

      ;; current node is archived
      (setq previous-node current-node)
      ;; Get parent node and continue the loop
      (setq current-node (js2-node-parent current-node)))

    (cond
     (rlt
      ;; Clean the final result
      (setq rlt (replace-regexp-in-string "^\\." "" rlt))
      (kill-new rlt)
      (message "%s => kill-ring" rlt))
     (t
      (message "No JSON path found!")))

    rlt))

Step 2, move the cursor over the key/value of the field and `M-x js2-print-json-path`.

My code is merged into js2-mode. You need only install latest js2-mode.

(apply #'derived-mode-p '(org-mode web-mode))

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