This binds `<leader>jq` to a command that pipes the current visual selection in evil-mode to `jq` and replaces it with the output.

pipe_to_jq.el

;; Taken from here: https://emacs.stackexchange.com/questions/34894/how-to-execute-a-command-on-the-selection-shell-command-on-region-in-evil-mode
(defun shell-command-on-region-and-select
    (start end command
           &optional output-buffer replace
           error-buffer display-error-buffer
           region-noncontiguous-p)
  "Wrapper for 'shell-command-on-region', re-selecting the output.

Useful when called with a selection, so it can be modified in-place"
  (interactive)
  (let ((buffer-size-init (buffer-size)))
    (shell-command-on-region
     start end command output-buffer replace
     error-buffer display-error-buffer
     region-noncontiguous-p)
    (setq deactivate-mark nil)
    (setq end (+ end (- (buffer-size) buffer-size-init)))
    (set-mark start)
    (goto-char end)
    (activate-mark)
    ;; needed for evil line mode
    (when (string= evil-state "visual")
      (when (eq (evil-visual-type) evil-visual-line)
        (evil-visual-select start end 'line)))))

(defun pipe-to-jq()
  (interactive)
  (shell-command-on-region-and-select
   (region-beginning) (region-end)
   "cat | jq ." nil t))

(evil-leader/set-key "jq" 'pipe-to-jq)