javascript function from python flask
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Upload new File</title>
</head>
<body onload="flashMessage()">
<script>
function flashMessage() {
if ("{{ flash_message }}" == "True") {
alert("[YOUR_MESSAGE_HERE]");
}
}
</script>
<h1>Upload new File</h1>
<form method=post enctype=multipart/form-data>
<input type=file name=file>
<input type=submit value=Upload>
</form>
</body>
</html>