Saving Data With localStorage

Every time someone refreshes your calculator, everything they entered disappears.

For calculators and generators that is fine because users fill them in fresh each time.

For trackers, habit apps, task managers, and anything where the user builds up data over time, the app is useless without memory.

localStorage is the solution. This lesson explains what it is, when to use it, and exactly how to tell Claude to implement it.

What localStorage is

localStorage is storage that lives inside the user's browser.

It is not on your server. It is not in a database. It is on their specific device in their specific browser. When they close Chrome and reopen it, localStorage is still there. When they turn off their computer and turn it back on, it is still there.

Think of it as the browser's permanent notebook.

What localStorage can and cannot do

It can save text, numbers, lists, and structured data. It persists between page refreshes and browser restarts. It can store up to around 5MB of data per site.

It cannot share data between different devices because a user's phone and laptop have separate localStorage. It cannot share data between different users. And it cannot store anything securely because this data can be read by anyone with access to the browser.

The rule of thumb: use localStorage for personal productivity tools, trackers, and preference settings. Do not use it for anything multi-user or sensitive.

When your app needs localStorage

Use it when you are building a to-do list where tasks must persist when the user returns, when you are building a habit tracker where streaks mean nothing if they reset daily, when you are building a content idea bank where users add ideas over time, when you want to remember a user's last inputs so they do not have to re-enter them, or when you want to save a quiz result so they can come back to see it.

Skip it when you are building a calculator where fresh inputs each time is the expected behaviour, when you are building a one-time generator where the user takes the output and leaves, or when resetting the app on each load is the intended experience.

How to tell Claude to use localStorage

For tracker and manager apps, add this section to your master build prompt:

DATA STORAGE — use localStorage for all data:

When user adds [AN ITEM], save it to localStorage immediately.
When the page loads, read from localStorage and display all saved [ITEMS].
When user deletes [AN ITEM], remove it from localStorage immediately.
[If applicable: When user marks [AN ITEM] as complete, update it in localStorage.]
localStorage key: "[YOUR-APP-NAME]-data"
Data format: JSON array of objects where each object contains: [LIST THE FIELDS]

The app should never lose user data on refresh, back button, or browser restart.

The reset and clear function

Any app that uses localStorage must have a way to clear the data.

Users may want to start fresh. They may be on someone else's computer. They may have made a mistake and want to undo everything.

Always tell Claude to include this:

• Add a "Clear All Data" button to the app.
• When clicked: show a confirmation dialog saying "Are you sure? This will delete all
• your data and cannot be undone."
• If the user confirms: clear all localStorage for this app and reset the view to the empty state.
• The button should be visible but not visually prominent. Use a small secondary style
• and place it below the main interface, not near the primary action buttons.

The refresh test

After Claude builds your localStorage-enabled app, run this test before anything else.

Open the app in the browser. Add some data such as a task, a habit entry, or a note depending on what the app stores. Press Ctrl+R on Windows or Cmd+R on Mac to refresh the page. Is the data still there?

If yes, localStorage is working correctly.

If no, send Claude this:

• The data is not persisting between page refreshes.
• The app should save to localStorage every time the user adds, edits, or deletes an item.
• On page load the app should check localStorage and populate the display with any saved data.
• Debug and fix the localStorage implementation in app.js.
• localStorage for calculator preferences (optional)
• Even for calculators you can improve the experience by remembering the user's last inputs so they do not have to re-enter everything each time they visit.
• When the user fills in any field and clicks Calculate, save all the input values
• to localStorage with the key "[app-name]-last-inputs".
• When the page loads, check if saved inputs exist in localStorage.
• If they do, pre-populate all fields with the saved values.
• Add a small "Using your saved inputs" notice above the form when this happens,
• with a link labelled "Clear saved inputs" that resets all fields to empty.

Your action right now

Decide whether your app needs localStorage.

If yes, add the localStorage section to your master build prompt right now using the template from this lesson.

If no, move to the next lesson which gives you a complete worked example for a calculator app.