Devtools console shows IEx console logs

add in config/dev.exs

  config :my_app, MyAppWeb.Endpoint,
    live_reload: [
      interval: 1000,
      patterns: [...],
+     web_console_logger: true
    ]

add in app.js

 + window.addEventListener("phx:live_reload:attached", ({detail: reloader}) => {
+   // enable server log streaming to client.
+   // disable with reloader.disableServerLogs()
+   reloader.enableServerLogs()
+ })

Source: elixirstreams