JavaScript Factorial Calculation

Codepen:

JavaScript Factorial Calculation

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Github: https://github.com/furkangulsen98/HTML-CSS-JAVASCRIPT/tree/master/factorial%20calculation

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HTML CODE:

<div class="container">
        <h2>factorial calculation</h2>
        <input id="number" type="number" />
        <button id="calc">CALCULATE</button>
    </div>

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CSS CODE:

        body {
            padding: 0;
            margin: 0;
            background-color: #333;
            font-family: 'Lucida Sans', 'Lucida Sans Regular', 'Lucida Grande', 'Lucida Sans Unicode', Geneva, Verdana, sans-serif;
        }
        .container {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%);
            background: #b72626;
            width: 400px;
            height: 200px;
            border-radius: 20px;
            box-shadow: 0px 0px 40px #000;
        }
        h2 {
            text-transform: uppercase;
            color: #dedede;
            text-align: center;
            text-shadow: 0px 0px 10px rgba(0, 0, 0, 1)
        }
        input {
            display: block;
            width: 75px;
            height: 75px;
            border: none;
            background: #dedede;
            border-radius: 50%;
            box-shadow: 0px 0px 20px #262626;
            margin: 0 auto;
            outline: none;
            font-size: 20px;
            font-weight: bold;
            font-family: 'Lucida Sans';
            text-align: center;
        }
        button {
            background: #000;
            padding: 8px;
            border: none;
            color: #fff;
            font-family: 'Lucida Sans';
            font-weight: bold;
            border-radius: 20px;
            margin-left: 150px;
            margin-top: 15px;
            outline: none;
            cursor: pointer;
            transition: .5s;
        }
        button:hover{
            background: #333;
        }

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JS CODE:

        let calc = document.getElementById("calc");

        calc.onclick = () => {
            const num = document.querySelector("#number").value;
            var arr = 1;
            for (var i = num; i > 0; i--) {
                arr *= i 
            }
            if (num < 0) {
                alert("negative numbers do not have a factorial")
            }
            alert(arr);
        }