Modal on page load

HTML

<div id="modal">
    <div class="modalconent">
         <h1></h1>

        <p>fasfsdfasfsfsdfsdfsdsffsd</p>
        <button id="button">Close</button>
    </div>
</div>

CSS

#modal {
    position: fixed;
    font-family: Arial, Helvetica, sans-serif;
    top: 0;
    left: 0;
    background: rgba(0, 0, 0, 0.8);
    z-index: 99999;
    height: 100%;
    width: 100%;
}
.modalconent {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background: #fff;
    width: 80%;
    padding: 20px;
}

JavaScript

function () {
    document.getElementById('button').onclick = function () {
        document.getElementById('modal').style.display = "none"
    };
};