What is the function of sessionStorage object in HTML? code example

Example 1: localStorage

// localStorage for objects, arrays or any data type
var obj = {
	firstName: "Bob",
    lastName: "Jeff",
    age: 13
}
localStorage.setItem("itemname", JSON.stringify(obj)); // Save the obj as string
var item = JSON.parse(localStorage.getItem("itemname")); 
// ^^ Parse string then set `item` as the obj

Example 2: javascript sessionStorage

var setsession = window.sessionStorage.setItem("animals", "cat");
	var getsession = window.sessionStorage.getItem("animals");
	console.log(getsession);

Example 3: local storage

As the answers here already talk about the coding aspect. I will talk about
the concept.
Local storage is basically an object stored in the specific browser you are 
using in that moment. And thus it is tied to that browser in that device. It's 
duration is infinite so it never expires

If you only want to store data that only lasts for that browser session(
starts when you open a window and ends when you close it) then the best choice
is sessionStorage