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Megan Sullivan

[Sketchnote] Web Storage APIs: How Browsers Store Data

November 29th, 2020

(1-minute read)

A sketchnote about localStorage and sessionStorage

Browsers have two built-in ways to store data: sessionStorage and localStorage.


  • Data stored there is cleared when the page session ends (i.e., the browser tab/window closes).
  • Each tab has its own sessionStorage object, independent from the one in other tabs.


  • Data stored there has no expiration time.
    • Exception: If you're in a private tab, then localStorage is cleared when the last private tab is closed.
  • Storage object is specific to the protocol. (HTTP object is separate from HTTPS.)

Common API

sessionStorage and localStorage both implement the Storage interface.

Both objects contain a key-value store, which is where data is kept. The keys and values are both type DOMString.


  • .length - The number of entries in the Storage object's key-value store.
    const numEntries = sessionStorage.length


  • .setItem(key, value) - Adds the key-value pair to the store.
    localStorage.setItem("key", "value")
  • .getItem(key) - Retrieves the value for the specified key. (Returns null if the key doesn't exist.)
    const username = sessionStorage.getItem("key")
  • .removeItem(key) - Removes the key-value pair for the specified key. (If the key doesn't exist, nothing happens.)
  • .clear() - Removes all key-value pairs from the store.

Additional Resources