SPFJS-Next (working title)

A modern re-creation of YouTube's SPF ("Structured Page Fragments") — a lightweight, framework-agnostic JavaScript library for fast same-origin page navigation: only the changed fragments of a page are fetched and applied, never the whole page.

This glossary inherits its baseline vocabulary from the original SPF domain (see research.md for the reference). Terms are being sharpened as the design is grilled out; unresolved or contested terms are marked.

Language

Navigation: Moving from one URL to another on the same origin without a full page load. Avoid: page transition, routing, SPA navigation

Fragment: A section of the page identified by an element ID whose content is replaced during a navigation. Avoid: region, component, slot, partial

Response: The data describing what changed during a navigation — title, head, per-fragment body, foot, attribute updates — in the SPF JSON wire format. Avoid: page data, payload, state

Part: One unit of a streamed Response — e.g. a single fragment's content, or the head/foot — processed as it arrives rather than after the whole response is received. Resolved: streamed as NDJSON parts over a single request.

Managed Resource: A script or stylesheet registered by a name so it loads once across navigations and can be swapped between versions. The counterpart of an unmanaged resource, which is re-executed on every navigation. Avoid: asset, dependency, bundle

Prefetch: Fetching a navigation's response (and priming its resources) before the user actually triggers the navigation. Avoid: preload, prerender (distinct browser primitives — terms not yet settled)

Response Cache: The local store of previously received Responses, used to avoid network requests on repeat and back/forward navigations. Avoid: HTTP cache, bfcache (different mechanisms)

New vs History Navigation: A new navigation is user- or API-initiated; a history navigation is back/forward. The cache treats them differently: a cached response is only eligible for history navigation unless unified. Resolved: this distinction is kept.

SPF Identifier: The URL marker (default ?spf=__type__, e.g. ?spf=navigate) that tells the server a request is an SPF request. Resolved: kept, with the response type negotiated via the Accept header.

SPF Link: An <a> or <form> element marked with the spf-link class (class name configurable) that triggers SPF navigation; spf-nolink opts an element out.

Module Specifier: A bare name or URL used to import a managed resource. Bare names resolve through the page's import map; absolute URLs import directly. Version switching happens by changing what a specifier resolves to, not by reloading the same URL.