<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<!--<link rel="stylesheet" href="1.css">-->
<style>
#txt {
display: none;
}
</style>
</head>
<body>
<div>
<input type="checkbox" id="check">
<label for="check">OK</label>
<input type="text" id="txt">
</div>
<!--<script type="text/javascript" src="1.js"></script>-->
<script>
document.getElementById('check').onclick = function() {
document.getElementById('txt').style.display = ['none', 'block'][+this.checked]
}
</script>
</body>
</html>