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