Code

<form action="#">
    <label for="name">Enter Number</label>
    <br />
    <input
        type="number"
        id="user_number"
        placeholder="Enter Number"
        required
        autocomplete="off"
    />
    <span id="result"></span>
    <br />
    <button type="submit" onclick="getFactorial()">Submit</button>
</form>
<script>
    const factorial = (n) =>{
        if(n == 0 || n == 1){
            return 1;
        }else{
            return n * factorial(n-1);
        }
    }
    const getFactorial = () => {
        let user_input = document.getElementById('user_number').value
        let result = document.getElementById('result')
        result.innerHTML = `<br /><label>Result : ${factorial(parseInt(user_input))}</label>`
    }
</script>