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    Migrating experience.js to the Optimization Web SDK

    Use this guide when a plain browser app or custom JavaScript adapter uses @ninetailed/experience.js and you want to move to @contentful/optimization-web.

    The legacy browser client owns plugin arrays, legacy storage keys, browser globals, and imperative event methods. The Web SDK uses one ContentfulOptimization instance for the browser runtime. Your app owns Contentful fetching, routing, rendering, consent records, identity policy, and any third-party forwarding.

    Follow the Web SDK integration guide for the target setup details.

    Gather these inputs:

    • The module that constructs Ninetailed, any injected instance, and every plugin passed to it.
    • Calls to page, track, identify, batch, reset, debug, flag reads, and manual rendering helpers.
    • Use of window.ninetailed, __nt_anonymous_id__, __anon_id, __nt_profile__, __nt_experiences__, __nt_changes__, __nt_debug__, __nt-consent__, or ntaid.
    • The app's consent and analytics policy.
    • The Contentful entries that were resolved through legacy mapped experiences.
    • The target Optimization clientId, environment, locale, and browser-visible config convention. These values identify the Contentful project and locale the browser runtime may call; keep secrets out of them and follow your bundler's public-variable convention.
    1. Migrate authored Contentful entries first when the app depends on legacy nt_* fields. Start with one authored all-visitors variant. See Migrating an experience.js Contentful model to Optimization.
    2. Install @contentful/optimization-web and create one Web SDK instance for the browser runtime.
    3. Replace the first page() call and entry resolution using the Web SDK guide.
    4. Replace identify, track, flags, reset, and consent with target SDK calls and app-owned policy.
    5. Move analytics, privacy, and preview plugins through Migrating experience.js plugins and preview.
    6. Remove @ninetailed/experience.js and legacy plugin packages after imports are gone.

    Start with the file that creates new Ninetailed(...) or injects an existing instance into a provider. List every plugin and every module that reads window.ninetailed. Legacy provider props did not re-create the client after construction, so treat client setup as a single runtime boundary rather than a reactive config surface.

    Also inventory persistence. Legacy browser storage and the ntaid cookie do not map directly to the target cookie name or consent model. Decide whether to reset visitor identity during migration or perform an app-owned continuity handoff.

    Create one ContentfulOptimization instance and reuse it. The Web SDK is ready immediately after construction, but optimization state is empty until an accepted event such as page() or identify() returns selections. An accepted event returns { accepted: true, data? }; stateful browser runtimes also expose accepted events on states.eventStream. Keep the order:

    1. Construct the instance.
    2. Apply the visitor's current consent defaults.
    3. Emit the page or identity event your policy allows.
    4. Resolve entries or read flags after state is available.

    Do not preserve legacy plugin arrays. Replace each plugin concern with the target SDK surface or an app-owned integration.

    Replace page, track, identify, and reset calls

    Map imperative calls by ownership:

    • page() and route tracking move to Web SDK event methods or trackCurrentPage().
    • identify({ userId, traits }) requires a user ID and returns an accepted or blocked result.
    • track({ event, properties }) owns application events.
    • reset() clears SDK profile state and the SDK-owned ctfl-opt-aid cookie, but it does not clear your consent record.

    The target consent model has two axes: event consent controls whether SDK events may be sent, and persistence consent controls whether the SDK may store the profile-id cookie. With the default pre-consent allow-list, page and identify can be admitted before explicit consent; set allowedEventTypes: [] if your policy requires every event to wait.

    Replace entry and flag rendering

    Stop feeding mapped experience configuration into browser rendering. Pass a fetched baseline entry to resolveOptimizedEntry() or use the Web Components path from the Web guide. When no selection exists, links are unresolved, or the payload uses all locales, the resolver returns the baseline.

    Replace legacy flag hooks with target flag reads. Stateful flag reads auto-attempt flag-view tracking when consent and profile state allow it; trackFlagView() is the explicit manual path.

    Use the Web guide's production checks, then add migration-specific checks:

    • One accepted page event populates selections before entry resolution.
    • Denied or undecided consent behaves according to your app policy.
    • Accepted events appear on states.eventStream; consent-blocked diagnostics appear on states.blockedEventStream.
    • No code reads window.ninetailed or legacy storage keys.
    • Run a browser route with a test all-visitors variant and verify variant rendering.
    • Repeat with consent denied and verify blocked diagnostics or baseline behavior.
    • Verify analytics forwarding, if any, dedupes by messageId.
    • Search for remaining @ninetailed/experience.js, window.ninetailed, and legacy storage keys.
    Symptom Check
    Entry resolution always returns baseline Confirm an accepted page or identify event returned selections before resolution, and confirm the Contentful payload contains linked Optimization entries.
    A second SDK instance throws Keep one Web SDK instance per browser runtime; destroy only during teardown.
    Vendor analytics stopped receiving events Replace legacy plugins with event-stream forwarding from the target SDK.
    Flags read but views are missing Confirm consent, profile availability, and whether you need explicit trackFlagView().

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