Tutorial: make clear warnings are to be found in dev console

I'm completely new to react and when I tried following https://reactjs.org/tutorial/tutorial.html , everything went smoothly except at the part before "Picking a Key" where it says

For now, we should see a list of the moves that have occurred in the game and a warning that says:

Warning: Each child in an array or iterator should have a unique “key” prop. Check the render method of “Game”.

I didn't realize codepen example had secretly overridden console.log to display the error msg in the page itself, so I thought I should be seeing the warning on my test page as well. I spent a long time looking for it, reexamining my code and setup, etc etc, and finally ended up asking and getting an answer on stackoverflow.

It's silly how the simplest things can become impossible hurdle for beginners who are learning on their own. I think a clarification that 1. for people following along with their own dev environment, this is to be found in the browser's developer console, and 2. the codepen example had changed console.log behavior but that's not the default to be expected.

Thank you very much.

React チュートリアルの「Picking a Key」の直前のセクションから始め、CodePen の例とローカル開発環境向けの手順を比較してください。警告はブラウザーの開発者コンソールに表示され、CodePen はそれをページに表示するために console.log を上書きしていることを明確にしてください。初心者がページ内で警告を探す可能性が低くなれば完了です。