let num = document.forms[0]["number"].value;
if (num <= 0){
alert("Количество не может быть меньше 1");
return false;}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<form name="myForm" action="send.php" method="POST" onsubmit="return validate()">
<input class="name" type="text" name="name" placeholder="Ваше имя"><br>
<input class="email" type="email" name="email" placeholder="Ваш email"><br>
<input type="number" name="number" placeholder="Your number"><br>
<input class="theme" type="text" name="theme" placeholder="Тема сообщения"><br>
<input class="submit" type="submit" name="submit" value="Send">
</form>
<script>
function validate() {
let name_ = Number(document.forms[0]["name"].value);
if (name_ === 2) {
alert("Поле \'Ваше имя\' не может быть пустым");
return false;
}
let email = document.forms[0]["email"].value;
if (email === "") {
alert("Поле \'Email\' не может быть пустым");
return false;
}
let num = document.forms[0]["number"].value;
if (num <= 0) {
alert("<0");
return false;
}
}
</script>
</body>
</html>