Set cookie on page load and hide div if cookie is present

Question

I have a site that shows a modal window when the page is loaded. I want this to only be shown to visitors the first time they come to the site however.

I have set JavaScript to add a cookie on page load but I want that cookie to be read (if present) and hide the modal (if cookie present).

I have the following Javascript (no jQuery):

<script language="JavaScript">
    if (document.cookie.indexOf('visited=true') == -1) {
        var oneyear = 1000*60*60*24*365;
        var expires = new Date((new Date()).valueOf() +  oneyear);
        document.cookie = "visited=true;expires=" +  expires.toUTCString();
}

//Check if cookie is present and hide myModal DIV

if (document.cookie.indexOf('visited=true') >= 0) {
    document.getElementById('myModal').modal.style.display = "none";
}
</script>

My Html is:

<div id="myModal" class="modal">
<!-- Modal content -->
    <div class="modal-content">
        <div class="content">
            <p>Text to be shown here</p>
            <button id="closeBtn">Close</button>
        </div>
    </div>
</div>

The cookie is being set on the page load, but it's not being read on second page load and hiding #myModal

** Update**

I have updated the above code, following the answers below but now I have got an error in the Console:

Uncaught TypeError: Cannot read property 'style' of null

Answer 1

Modify this.You are checking string ( visited=true= ) wrong. Set it to visited=true or visited= is the best option as per me

if (document.cookie.indexOf('visited=') >= 0) {
    document.getElementById('myModal').modal.style.display = "none";
}

Answer 2

You are checking the wrong string :

if (document.cookie.indexOf('visited=true=') >= 0) {

should be

if (document.cookie.indexOf('visited=true') >= 0) {

Answer 3

Could it be the .modal. that needs to be removed? Ie change

if (document.cookie.indexOf('visited=') >= 0) {
    document.getElementById('myModal').modal.style.display = "none";
}

to:

if (document.cookie.indexOf('visited=') >= 0) {
    document.getElementById('myModal').style.display = "none";
}