Introduce `fetchpriority` for Scripts and Script Modules #8815

westonruter · 2025-05-19T05:14:29Z

Add new fetchpriority argument to go alongside the loading strategy (async & defer) when registering scripts via wp_register_script()/wp_enqueue_script(). This is a follow-up to the script loading strategies introduced in Core-12009.
Add a new 5th$args parameter to wp_register_script_module()/wp_enqueue_script_module() (and their corresponding methods) to mirror the same 5th $args parameter used in wp_register_script()/wp_enqeue_script(). This $args parameter same fetchpriority argument introduced for non-script modules. (Note that module scripts have a defer loading strategy by default, so adding support for a strategy would only be useful for async. This is not in the scope of this PR.) This will apply the fetchpriority attribute both to the SCRIPT[type="module"] tags as well as the LINK[rel="modulepreload"] tags for static import dependencies.
The default fetchpriority value for both scripts and script modules is auto.
When the fetchpriority attribute is emitted on SCRIPT and LINK tags, it is omitted if the value is auto since this is the default value.
The comment-reply script is given an explicit fetchpriority of low. While the script is already registered with the async loading strategy which Chrome causes the resource to be downloaded with a low priority by default, this is not the case for Safari or Firefox which use a default medium/normal priority. So the explicit low priority reduces the chance that the loading of the comment-reply script will interfere with the loading of resources needed in the critical rendering path (e.g. an LCP image element).
Script modules used for the view module scripts for blocks with Interactivity API support are given an explicit fetchpriority of low. The very first requirement/goal defined for the Interactivity API was for server-side rendering, therefore blocks should not depend on the view module script for initial rendering.
Tests have been written for the changes both to scripts and script modules, but the tests for the latter have additional changes since deficiencies were discovered (e.g. duplicate array keys, and global functions were untested).

Note

If Gutenberg is active, you won't see any changes without WordPress/gutenberg#70173 also checked out.

Trac ticket: https://core.trac.wordpress.org/ticket/61734

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

github-actions · 2025-05-19T05:26:54Z

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…odules

swissspidy · 2025-05-20T15:54:37Z

src/wp-includes/blocks.php

@@ -175,11 +175,18 @@ function register_block_script_module_id( $metadata, $field_name, $index = 0 ) {
 	$block_version       = isset( $metadata['version'] ) ? $metadata['version'] : false;
 	$module_version      = isset( $module_asset['version'] ) ? $module_asset['version'] : $block_version;

+	$args = array();
+	if ( isset( $metadata['supports']['interactivity'] ) && $metadata['supports']['interactivity'] ) {
+		// TODO: Add ability for the fetchpriority to be specified in block.json for the viewScriptModule. In wp_default_script_modules() the fetchpriority defaults to low since server-side rendering is employed for core blocks, but there are no guarantees that this is the case for non-core blocks. That said, viewScriptModule entails Interactivity API, following the pattern from core it _should_ be SSR'ed and that is why this is the default for when the block supports interactivity.


Maybe that property in block.json should become an object instead of a string. In the future we might need other properties in there too.

cc @gziolo

Absolutely! See below 😄 #8815 (comment)

swissspidy · 2025-05-20T15:55:09Z

src/wp-includes/blocks.php

@@ -243,6 +250,7 @@ function register_block_script_handle( $metadata, $field_name, $index = 0 ) {
 	$script_args         = array();
 	if ( 'viewScript' === $field_name && $script_uri ) {
 		$script_args['strategy'] = 'defer';
+		// TODO: There needs to be a way to specify that a script defined in a module is safe to use fetchpriority=low. Perhaps the viewScript should not only allow a handle string or a `file:./foo.js` string, but allow an an array of params like { "href": "./foo.js", "fetchpriority": "low" }. This would allow for more metadata to be supplied, like the script loading strategy. Related: <https://core.trac.wordpress.org/ticket/56408> and <https://core.trac.wordpress.org/ticket/54018>.


Oh, yes, exactly what I meant above 😅

src/wp-includes/script-modules.php

src/wp-includes/class-wp-script-modules.php

Co-authored-by: Pascal Birchler <[email protected]>

src/wp-includes/blocks.php

Co-authored-by: Luis Herranz <[email protected]>

westonruter · 2025-05-21T15:27:19Z

src/wp-includes/class-wp-script-modules.php

+ * @phpstan-type ScriptModule array{
+ *     src: string,
+ *     version: string|false|null,
+ *     enqueue: bool,
+ *     dependencies: array<array{ id: string, import: 'dynamic'|'static' }>,
+ *     fetchpriority: 'auto'|'low'|'high',
+ * }


I added these @phpstan annotations for myself as a safeguard while developing this patch. If others don't want PHPStan annotations in core, I'll remove prior to commit.

westonruter added 3 commits May 18, 2025 20:10

Output script modules with fetchpriority=low

dbcd32c

Fix duplicate array keys in data_special_chars_script_encoding

5207c27

Add ability to set fetch priority for script modules

088366d

westonruter added 8 commits May 19, 2025 12:26

Add fetchpriority support for non-module scripts

9eab85d

Set fetchpriority=low on comment-reply script

d9c4037

Use auto as default fetchpriority for script modules

4f82a4f

Avoid printing fetchpriority attribute when auto

f2cd9be

fixup! Use auto as default fetchpriority for script modules

ff4fd36

Use HTML Tag Processor to parse import map

9edbe4c

Ensure parity in args between class methods and global function aliases

021fe74

Use fetch priority low by default for Interactivity API view script m…

34ef7fa

…odules

westonruter changed the title ~~Use fetchpriority=low by default for script modules and add ability to override~~ Introduce fetchpriority for Scripts and Script Modules May 20, 2025

swissspidy reviewed May 20, 2025

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swissspidy reviewed May 20, 2025

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swissspidy reviewed May 20, 2025

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src/wp-includes/class-wp-script-modules.php Show resolved Hide resolved

Add missing since tag

e54274a

Co-authored-by: Pascal Birchler <[email protected]>

westonruter mentioned this pull request May 21, 2025

Add fetchpriority=low support to script modules WordPress/gutenberg#70173

Draft

luisherranz reviewed May 21, 2025

View reviewed changes

src/wp-includes/blocks.php Outdated Show resolved Hide resolved

Account for full block.json schema when checking for interactivity

cc1d909

Co-authored-by: Luis Herranz <[email protected]>

westonruter commented May 21, 2025

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Introduce `fetchpriority` for Scripts and Script Modules #8815

Introduce `fetchpriority` for Scripts and Script Modules #8815

westonruter commented May 19, 2025 •

edited

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github-actions bot commented May 19, 2025

swissspidy May 20, 2025

westonruter May 21, 2025

swissspidy May 20, 2025

westonruter May 21, 2025

Introduce fetchpriority for Scripts and Script Modules #8815

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Introduce fetchpriority for Scripts and Script Modules #8815

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Introduce `fetchpriority` for Scripts and Script Modules #8815

Introduce `fetchpriority` for Scripts and Script Modules #8815

westonruter commented May 19, 2025 •

edited

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