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Repository Boundary

Restore persisted preferences only when their schema and ownership scope are valid. This pattern belongs to Design Engineering and adds no Layout CSS or shared store implementation.

Reusable Method

Use for intentional persistence of non-sensitive preferences or drafts with a declared retention policy. Keep state in memory if cross-session recovery has no user value.

State Model And Ownership

The persistence adapter owns a versioned envelope and scope key. Decoding validates shape and allowed values before applying. This example rejects old versions; migration is a separate explicit function. Current user edits outrank a late restore.

Transitions

Minimal Executable Example

Run this standalone JavaScript block with Node.js 22 or newer, or run all pattern examples from a repository checkout with npm run test:state-management. The assertions exercise the local model, not a browser or backend.

import assert from "node:assert/strict";
function restore(raw, scope) {
  try {
    const record = JSON.parse(raw);
    if (!record || record.version !== 1 || record.scope !== scope) return "compact";
    return ["compact", "comfortable"].includes(record.density) ? record.density : "compact";
  } catch { return "compact"; }
}
const raw = JSON.stringify({ version: 1, scope: "account:A", density: "comfortable" });
assert.equal(restore(raw, "account:A"), "comfortable");
assert.equal(restore(raw, "account:B"), "compact");
assert.equal(restore("{broken", "account:A"), "compact");
assert.equal(restore(JSON.stringify({ version: 0 }), "account:A"), "compact");
let editRevision = 0, density = "compact";
const restoreStartedAt = editRevision;
density = "comfortable"; editRevision++;
if (editRevision === restoreStartedAt) density = restore(null, "account:A");
assert.equal(density, "comfortable");

What Breaks If Removed

Blind JSON parsing can crash startup. An unscoped storage key leaks settings across accounts. Applying a delayed restore unconditionally overwrites edits made while storage was loading.

Composition And Substitution

Use identity reset for account changes. Persist only source values from derived state; do not restore loading flags, request IDs, or obsolete cached errors.

Opinionated Guidance

Choose migration, expiry, and multi-tab conflict policies before persisting. A version field without a decoder or migration rule is not a compatibility strategy.

Platform-Specific Guidance

Browser storage access and writes can throw or be unavailable; guard the adapter as well as parsing. Server rendering cannot read browser storage. Define hydration fallback and cross-tab event handling separately.

Platform reference: MDN API documentation, checked 2026-09-16. This supports the named browser boundary; the model and product policy remain local synthesis.

Unsupported Absolutes

Persistence does not guarantee availability, confidentiality, or synchronization across devices.

Verification Contract

• Decode corrupt, missing, old-version, and wrong-account records; use valid defaults.
• Edit before an asynchronous restore finishes; preserve the edit.
• Simulate unavailable storage and quota failure; keep the screen usable.

Record actual outcomes separately using the verification matrix.

Source, License, And Attribution

Locally authored synthesis and examples. No upstream implementation is copied. These experimental contracts describe consumer-owned behavior; review when a failure, ownership change, or platform change invalidates an assumption.

IA Navigation

Parent: State Management. Next: Continue the state management route.