Every browser has a JavaScript console. Usually, you can open this console with a simple keyboard shortcut:
– Chrome: Press ⌘
+⌥
+J
(on a Mac) or Ctrl
+Shift
+J
(on Windows).
– Firefox: Press ⌘
+⌥
+K
(on a Mac) or Ctrl
+Shift
+K
(on Windows).
– For other browsers or more details, read the official guide on wordpress.org