What is the best practice for executing a JavaScript function automatically when a web page finishes loading?

I am building a dynamic dashboard and I need to trigger an initialization function—like fetching data or setting up event listeners—as soon as the page is ready. I’ve tried using the onload attribute in the <body> tag, but it feels outdated and clutters my HTML. Is there a more modern way to do this using pure JavaScript (Vanilla JS) or a specific React hook? I want to ensure the DOM is fully constructed before the function runs to avoid "null" reference errors when accessing elements.