Using js to implement simple switch light code

<button>Turn on/off light</button>

    <script>

        // window.onload is the window loading event, which can be used to write code to the element window.addEventListener('load', function () {

            var btn = document.querySelector('button');

            // Define a variable to determine whether the light is on or off var flag = 0;

            btn.onclick = function () {

                if (flag == 0) {

                    document.body.style.backgroundColor = 'black';

                    flag = 1;

                } else {

                    document.body.style.backgroundColor = 'pink';

                    flag = 0;

                }

            }

        })

    </script>

<script>

        var btn = document.querySelector('button');

        var flag = 0;

        btn.onclick = function () {

            if (flag == 0) {

                document.body.style.backgroundColor = 'black';

                flag = 1;

            } else {

                document.body.style.backgroundColor = 'pink';

                flag = 0;

            }

        }

    </script>