[wp-trac] [WordPress Trac] #62709: Script modules integration with wp_resource_hints

[WordPress Trac]

#62709: Script modules integration with wp_resource_hints
---------------------------+--------------------------------------
 Reporter:  jonsurrell     |       Owner:  (none)
     Type:  enhancement    |      Status:  new
 Priority:  normal         |   Milestone:  7.1
Component:  Script Loader  |     Version:  6.5
 Severity:  minor          |  Resolution:
 Keywords:  needs-patch    |     Focuses:  javascript, performance
---------------------------+--------------------------------------

Comment (by sanket.parmar):

 Thanks for filing this — the gap is clear. Here are some thoughts on a
 possible approach.

 == The Core Issue

 `wp_dependencies_unique_hosts()` currently only loops over `$wp_scripts`
 and `$wp_styles`:

 {{{
 #!php
 foreach ( array( $wp_scripts, $wp_styles ) as $dependencies ) {
     // ...
 }
 }}}

 `WP_Script_Modules` is a completely separate system and is never
 consulted, so no `dns-prefetch` hints are emitted for external hosts used
 by script modules.

 == Possible Approaches

 === Option A: Hook into `wp_resource_hints` filter from within
 `WP_Script_Modules`

 One approach could be to add a new public method to `WP_Script_Modules` —
 something like `filter_resource_hints( array $urls, string $relation_type
 ): array` — and register it from within `add_hooks()`:

 {{{
 #!php
 add_filter( 'wp_resource_hints', array( $this, 'filter_resource_hints' ),
 10, 2 );
 }}}

 The method would:
  * Only act when `$relation_type === 'dns-prefetch'`
  * Walk the queue and all dependencies (both static and dynamic)
  * Parse the host from each `src` URL, filtering out same-site hosts
  * Merge the resulting hosts into `$urls` and return

 This keeps all the logic inside `WP_Script_Modules` and avoids changing
 the signature or behavior of `wp_dependencies_unique_hosts()`.

 === Option B: Extend `wp_dependencies_unique_hosts()` directly

 Alternatively, `wp_dependencies_unique_hosts()` could be extended to also
 inspect `$wp_script_modules`. This would require exposing a getter on
 `WP_Script_Modules` (e.g. `get_registered_srcs()` or similar), which may
 partly overlap with the work in #60597.

 == A Note on Redundancy

 As @swissspidy raised — `print_script_module_preloads()` already emits
 `rel="modulepreload"` for ''static'' dependencies, and `modulepreload`
 implies a full connection (DNS + TCP + TLS). Adding `dns-prefetch` on top
 of those may be redundant.

 That said, two cases still seem clearly worth targeting:
  1. '''Dynamic dependencies''' — declared as `'import' => 'dynamic'`,
 these are never preloaded and currently receive no performance hint at
 all.
  2. '''Classic themes''' — `wp_resource_hints` fires in `wp_head` at
 priority 2, while all module-related tags fire in `wp_footer` on classic
 themes. Even for static deps, an early `dns-prefetch` in the head could
 give the browser a head start before any module tag is encountered.

 Option A could optionally be scoped to only emit `dns-prefetch` for
 dynamic-dependency hosts (skipping those already covered by
 `modulepreload`), though that could also be left as a follow-up to keep
 the initial patch simpler.

 == Tests

 Any patch would likely need companion PHPUnit tests, probably alongside
 `tests/phpunit/tests/general/wpResourceHints.php`, covering:
  * Dynamic-dependency hosts appear in dns-prefetch
  * Static-dependency hosts (already covered by modulepreload) are either
 included or explicitly excluded, depending on the chosen design
  * Same-site hosts are not included

 Happy to help put together a patch if this direction looks reasonable.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/62709#comment:7>
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