addWindow.html

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Add Shopping List Item</title>
    <!-- MaterializeCSS -->
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/materialize/1.0.0-rc.2/css/materialize.min.css">
</head>
<body>
    <div class="container">
        <form>
            <div>
                <label>Enter an Item</label>
                <!-- As soon as window loads, cursor will be auto focussed on input -->
                <input type="text" id="item" autofocus>
            </div>

            <button class="btn waves-effect waves-light" type="submit" name="action">
                Add Item
            </button>
        </form>
    </div>

    <script>
        // ipcRenderer => will pass item's value to MainWindow through main.js
        const electron = require("electron");
        const {ipcRenderer} = electron;

        const form = document.querySelector("form");
        // When form is submitted
        form.addEventListener('submit', submitForm);

        function submitForm(event){
            // Whenever we submit a form, it's automatically gonna be submitted to a file
            // To prevent this default behavior from happening
            event.preventDefault();
            
            const item = document.querySelector("#item").value;
            if(item){
                ipcRenderer.send('item:add', item);
            }
        }
    </script>

</body>
</html>