operators - || in javascript

Question

Can somebody please tell me what || this doing here

(function()
{
window.myapp = window.myapp || {};
window.myapp.lang = window.myapp.lang || {};
myapp.lang.Extend = function(subClass, superClass)
{
subClass.prototype = new superClass();
};
})();

Answer 1

window.myapp = window.myapp || {};

It means: create window.myapp as an empty object if it does not already exist.