Easy way to check whether customer logged in or not via customer Js component in phtml file.

<div data-bind="scope: 'customer'">
    <!-- ko if: customer().fullname -->
        Logged In customer
    <!-- /ko -->
    <!-- ko ifnot: customer().fullname -->
         Notlogged In customer
    <!-- /ko -->
</div>

<script type="text/x-magento-init">
{
    "*": {
        "Magento_Ui/js/core/app": {
            "components": {
                "customer": {
                    "component": "Magento_Customer/js/view/customer"
                }
            }
        }
    }
}
</script>

If you want to use it in the js file

define([
    'uiComponent',
    'Magento_Customer/js/model/customer'
], function (
    Component,
    customer
) {
    'use strict';

    return Component.extend({
        /**
         * Check if customer is logged in
         *
         * @return {boolean}
         */
        isLoggedIn: function () {
            return customer.isLoggedIn();
        }
    });
});

You can call in your related HTML template.

<!-- ko if: isLoggedIn() -->
    <div>Customer is logged in</div>
<!-- /ko -->