Javascript: Closures, callback, context

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time.

window.onload = function(){
    var item = document.querySelector(".items .item");
    item onClick = activeItem;

    function activeItem() {

In this example inner function activeItem() have access to global variable item. This called closure – when inner function have access to all variables, defined in global function.