The length read-only property of the Storage interface returns the number of data items stored in a given Storage object.

The number of items stored in the Storage object.

The following function adds three data items to the local storage for the current domain, then returns the number of items in the storage:

function populateStorage() {
  localStorage.setItem("bgcolor", "yellow");
  localStorage.setItem("font", "Helvetica");
  localStorage.setItem("image", "cats.png");

  return localStorage.length; // Should return 3
}

• Using the Web Storage API