DOM Проверьте флажок
document.getElementById("myCheck").checked = true;
Angry Alligator
document.getElementById("myCheck").checked = true;// Check
document.getElementById("checkbox").checked = true;
// Uncheck
document.getElementById("checkbox").checked = false;<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>JavaScript Checkbox</title>
</head>
<body>
<label for="accept">
<input type="checkbox" id="accept" name="accept" value="yes"> Accept
</label>
<script>
const cb = document.querySelector('#accept');
console.log(cb.checked); // false
</script>
</body>
</html>
Code language: HTML, XML (xml)