How to allow only alphabets in Input Field in Javascript

<!DOCTYPE html>
<html lang="en">
<head>
</head>

<body>
    <div id="inputContainer">
        <h1>Javascript only 1 space and Alphabets Allowed</h1>
        <label for="inputField">Enter text:</label>
        <input type="text" id="inputField">
        <button onclick="validateInput()">Validate</button>
        <p id="result"></p>
    </div>

function validateInput() {
            let userInput = document.getElementById("inputField").value;
            let resultElement = document.getElementById("result");

if (isValidInput(userInput)) {
                resultElement.innerText = "Input is valid";
                resultElement.className = "valid";
            } else {
                resultElement.innerText = "Invalid input";
                resultElement.className = "invalid";
            }
        }

</script>
    <style>
        body {
            font-family: Arial, sans-serif;
            background-color: #f4f4f4;
            margin: 0;
            display: flex;
            align-items: center;
            justify-content: center;
            height: 100vh;
        }
    
        #inputContainer {
            text-align: center;
            background-color: #fff;
            padding: 20px;
            border-radius: 8px;
            box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
        }
    
        label {
            font-size: 18px;
            display: block;
            margin-bottom: 10px;
        }
    
        input {
            padding: 10px;
            font-size: 18px;
            width: 250px;
            margin-bottom: 15px;
            border: 2px solid #3498db;
            border-radius: 5px;
            outline: none;
            transition: border-color 0.3s ease-in-out;
        }
    
        input:focus {
            border-color: #2ecc71;
        }
    
    
        button {
            padding: 10px 20px;
            font-size: 16px;
            background-color: #4caf50;
            color: #fff;
            border: none;
            border-radius: 4px;
            cursor: pointer;
        }
    
        button:hover {
            background-color: #45a049;
        }
    
        #result {
            font-size: 18px;
            margin-top: 10px;
            color: black;
        }
    
        #result.valid {
            color: green;
        }
    
        #result.invalid {
            color: red;
        }
    </style>
</body>

</html>