One way is to insert a loading animation in index.html
eg.
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div id="content">
<div id="loader" />
</div>
<script src="build/app.js"></script>
</body>
</html>
Then, after the bundle.js file loads, it will be displayed in the content, and the download animation will disappear.
EDIT:
webapp.
https://developers.google.com/web/updates/2015/10/splashscreen
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