Vanilla JavaScript

// Select elements
var elements = document.querySelectorAll('.item');
for (var i = 0; i < elements.length; i++) {
    elements[i].style.display = 'none';
}

// Add event listener
document.getElementById('btn').addEventListener('click', 
    function(e) {
        e.preventDefault();
        // do stuff
    }
);

// AJAX request
var xhr = new XMLHttpRequest();
xhr.open('GET', '/api/data', true);
xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
        var data = JSON.parse(xhr.responseText);
        console.log(data);
    }
};
xhr.send();