How to return with an error Alert, when you do not enter a value in the variable?

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I was doing some exercises to practice Javascript, when I came across a doubt, how do I fire an error Alert when at least one of the variables receives a string or no value?

That’s the code I created.

<!DOCTYPE html>
<html lang="pt-br">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Diferença</title>
    <style>
        body{
            font: normal 80px Arial;
        }
    </style>
</head>
<body>
    <script>
        var A = Number.parseInt(window.prompt('Digite o valor de A:'))
        var B = Number.parseInt(window.prompt('Digite o valor de B:'))
        var C = Number.parseInt(window.prompt('Digite o valor de C:'))
        var D = Number.parseInt(window.prompt('Digite o valor de D:'))
        var dif = (A*B)-(C*D)
        document.write(`DIFERENÇA = ${dif}`)
    </script>
</body>
</html>
javascript

  • Beyond the isNaN you should also compare with empty string if (a === ''), because when the user does not type anything, this value is an empty string.

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