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Digital · Guided lesson

Define the browser storage boundary

About 29 minutes · 7 reading + 22 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
See this course’s project requirements

A fictional local reading-list app with versioned saved data, explicit browser-only wording, validated JSON export/import and corrupt-save recovery tests.

Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.

What to include

  • Document the exact storage origin and distinguish reload persistence from account synchronization.
  • Retain the validated envelope format and prove an invalid replacement leaves the earlier record intact.
  • Demonstrate a rejected corrupt load and a failed write without silently discarding prior data.
  • Create and inspect an actual JSON export and state its exact contents.
  • Produce a valid/invalid import matrix proving rejected content retains the previous list.
  • Deliver a two-tab observation or truthful unavailable note, alongside the validated backup/recovery project.

Review and revise

Use these criteria to check your work and choose what to improve.

  • The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
  • Normal, boundary and rejected cases are recorded with expected versus observed results.
  • The changed case has a correct explanation and does not rely on hidden state.
  • The project includes source, outputs, instructions and honest tool, privacy and verification limits.
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Before you begin

The idea in everyday life

Use the supplied reading.html, reading-store.mjs and test-reading.mjs with the invented title Moon Garden. Serve the folder at http://localhost:8765 using python3 -m http.server 8765 --bind 127.0.0.1.

The idea

localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

Follow along

Define the browser storage boundary — original worked practice

localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

Use the supplied reading.html, reading-store.mjs and test-reading.mjs with the invented title Moon Garden. Serve the folder at http://localhost:8765 using python3 -m http.server 8765 --bind 127.0.0.1.

  1. 1. Step 1

    Open reading.html through the local server; read the browser-only status before entering a title.

    You should see: The app identifies local browser storage.

    localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

  2. 2. Step 2

    Add Moon Garden and reload the same localhost address.

    You should see: The title loads if that browser allows storage.

    localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

  3. 3. Step 3

    Change only the port in the address by using a separately started server at 8766; compare the empty list there.

    You should see: Different port means a different origin and separate storage.

    localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

  4. 4. Step 4

    Record the origin, browser, observed reload, and unavailable tests.

    You should see: An origin/storage scope record.

    localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup.

Now change one thing

A reading record was saved under http://localhost:8765/index.html. Change only the path to http://localhost:8765/reading.html, keeping scheme, host and port. Are these addresses the same origin?

Your work

What to finish

Document the exact storage origin and distinguish reload persistence from account synchronization.

Common mistake

Watch for this

Promising cross-device saving from a localStorage write.

How can I tell I understand it?

Demonstrate the rule: localStorage belongs to an origin and browser profile; it is not account storage or a guaranteed backup. Use the supplied variant to distinguish a changed result from a broken implementation.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1Why may Moon Garden disappear when the local app uses a different port?
2Which saving promise matches this reading app?
3Why serve reading.html instead of relying on file: behavior?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
1
Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

Explore the references behind this lesson.

developer.mozilla.org — localStorage
Source & access notes · checked 2026-10-01

Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.

Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.

Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.

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