magento2 check if customer is logged in or not in knockout template

You need to pass dependency of customer object, 'Magento_Customer/js/model/customer',

define(
[
    'ko',
    'jquery',   
    'Magento_Customer/js/model/customer',
    'mage/translate'
],
function (ko,jquery, customer,$t) {
    'use strict';
    return Component.extend({        
        isCustomerLoggedIn: customer.isLoggedIn, /*return  boolean true/false */
        initialize: function() {
            this._super();
            /* check using below method */
            var isLoggedIn = this.isCustomerLoggedIn();
        }
    });
}

Now you can check it using customer.isLoggedIn() returns boolean value.

define(
[
    'ko',
    'jquery',
    'Magento_Ui/js/form/element/abstract',
    'mage/url',
    'Magento_Customer/js/customer-data',    
    'Magento_Customer/js/model/customer',
    'mage/translate'
],
function (Component,storage,ko,jquery,Abstract, url, customerData, customer,$t) {
    'use strict';

    return Component.extend({
        message: function() {
        loggedMessage : $t('Welcome, %1!')
        },
        htmlLoggedMessage: ko.observable(),
        isLoggedIn: ko.observable(),
        isCustomerLoggedIn: customer.isLoggedIn,
        customer: ko.observable({}),
        initialize: function() {
            this._super();
            if(this.isCustomerLoggedIn()) {
                this.isLoggedIn(true);
            }
            this.checkCustomerLocalStorage();
        },
        /**
         * Check customer localstorage
         */
        checkCustomerLocalStorage: function () {
            var self = this;
            var time = setInterval(function () {
                self.customer = customerData.get('customer');
                if (localStorage["mage-cache-storage"] != '{}') {
                    clearInterval(time);
                }
                if (self.customer().fullname) {
                    var name = self.customer().fullname;
                    var message = self.message.loggedMessage.replace('%1', name);
                    self.htmlLoggedMessage(message);
                }
            }, 1000);
        }
    });
}