/code/javascript/run.html

<!DOCTYPE html>
<html>
<head>
	<title>Text editor</title>
	<style type="text/css">
		* { box-sizing: border-box; }
		html, body, textarea {
			margin: 0; border: 0; padding: 0;
			height: 100vh; width: 100vw;
		}
		textarea { top: 0; left: 0; margin-bottom: 2em; }
		textarea, button { position: absolute }
		button { display: block; bottom: 0; right: 0; }
	</style>
	<script type="text/javascript">
onload = function () {
	document.querySelector("button").onclick =
		() => eval(document.querySelector("textarea").value);
}
	</script>
</head>
<body><textarea></textarea><button>go</button></body>
</html>
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