The WordPress coreCoreCore is the set of software required to run WordPress. The Core Development Team builds WordPress. development team builds WordPress! Follow this site for general updates, status reports, and the occasional code debate. There’s lots of ways to contribute:
Found a bugbugA bug is an error or unexpected result. Performance improvements, code optimization, and are considered enhancements, not defects. After feature freeze, only bugs are dealt with, with regressions (adverse changes from the previous version) being the highest priority.?Create a ticket in our bug tracker.
The REST APIREST APIThe REST API is an acronym for the RESTful Application Program Interface (API) that uses HTTP requests to GET, PUT, POST and DELETE data. It is how the front end of an application (think “phone app” or “website”) can communicate with the data store (think “database” or “file system”) https://developer.wordpress.org/rest-api/. will see a lot of changes in WordPress 5.5. In an effort to explain each change adequately, a number of these were split out and covered by other dev notesdev noteEach important change in WordPress Core is documented in a developers note, (usually called dev note). Good dev notes generally include:
a description of the change;
the decision that led to this change
a description of how developers are supposed to work with that change.
Dev notes are published on Make/Core blog during the beta phase of WordPress release cycle. Publishing dev notes is particularly important when plugin/theme authors and WordPress developers need to be aware of those changes.In general, all dev notes are compiled into a Field Guide at the beginning of the release candidate phase..
Below are some other noteworthy changes that deserve a call out.
Discoverable REST resource links
To aid automated and human discovery of the REST API, a link was added in the <head> of the document and as a LinkheaderHeaderThe header of your site is typically the first thing people will experience. The masthead or header art located across the top of your page is part of the look and feel of your website. It can influence a visitor’s opinion about your content and you/ your organization’s brand. It may also look different on different screen sizes. to the REST route for the currently queried document in .
For example, in the <head> of this post, the following <link> appears.
For custom post types, only routes using the built-in WP_REST_Posts_Controller controller class will work by default. For custom controllers, the rest_route_for_postfilterFilterFilters are one of the two types of Hooks https://codex.wordpress.org/Plugin_API/Hooks. They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output. can be used to supply the correct route.
In , the Link header was added to the list of exposed CORS response headers. Previously, only the X-WP-Total and X-WP-TotalPages headers were exposed which means that clients would have to manually construct the URLURLA specific web address of a website or web page on the Internet, such as a website’s URL www.wordpress.org to implement pagination instead of using the prev and next Links.
In , the Content-Disposition, Content-MD5 and X-WP-Nonce were added to the list of allowed cors request headers. The Content-Disposition and Content-MD5 headers allow for easier file uploading across domains by using a File/Blob object directly. The X-WP-Nonce header is allowed for making authenticated cross-origin and same-origin requests consistently.
Two filters are introduced, rest_exposed_cors_headers and rest_allowed_cors_headers to simplify the process of plugins modifying the list of cors headers.
The REST API treats routes without a permission_callback as public. Because this happens without any warning to the user, if the permission callback is unintentionally omitted or misspelled, the endpoint can end up being available to the public. Such a scenario has happened multiple times in the wild, and the results can be catastrophic when it occurs.
In  a _doing_it_wrong notice has been added when a permission callback is omitted. For REST API routes that are intended to be public, it is recommended to set the permission callback to the __return_true built in function.
So for instance, this route registration will cause the following warning to appear.
When building a REST API route, it is important for the route callback (and permission_callback) to return data instead of directly sending it to the browser. This ensures that the additional processing that the REST API server does, like handling linking/embedding, sending headers, etc… takes place.
// This is incorrect.
echo wp_json_encode( $data );
// And this.
wp_send_json( $data );
// And this.
wp_send_json_error( 'My Error' );
// Instead do this.
return new WP_REST_Response( $data );
// Or this.
return new WP_Error( 'my_error_code', 'My Error Message' );
After , a _doing_it_wrong notice is now issued when any of the wp_send_json family of functions are used during a REST API request.