What is a storage event and its event handler

The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below

window.onstorage = functionRef;

Let's take the example usage of onstorage event handler which logs the storage key and it's values

window.onstorage = function (e) {
console.log(
"The " +
e.key +
" key has been changed from " +
e.oldValue +
" to " +
e.newValue +
"."
);
};