wp_resource_hints()

Prints resource hints to browsers for pre-fetching, pre-rendering and pre-connecting to web sites.


Description Description

Gives hints to browsers to prefetch specific pages or render them in the background, to perform DNS lookups or to begin the connection handshake (DNS, TCP, TLS) in the background.

These performance improving indicators work by using <link rel"…">.


Top ↑

Source Source

File: wp-includes/general-template.php

function wp_resource_hints() {
	$hints = array(
		'dns-prefetch' => wp_dependencies_unique_hosts(),
		'preconnect'   => array(),
		'prefetch'     => array(),
		'prerender'    => array(),
	);

	/*
	 * Add DNS prefetch for the Emoji CDN.
	 * The path is removed in the foreach loop below.
	 */
	/** This filter is documented in wp-includes/formatting.php */
	$hints['dns-prefetch'][] = apply_filters( 'emoji_svg_url', 'https://s.w.org/images/core/emoji/13.0.0/svg/' );

	foreach ( $hints as $relation_type => $urls ) {
		$unique_urls = array();

		/**
		 * Filters domains and URLs for resource hints of relation type.
		 *
		 * @since 4.6.0
		 *
		 * @param array  $urls          URLs to print for resource hints.
		 * @param string $relation_type The relation type the URLs are printed for, e.g. 'preconnect' or 'prerender'.
		 */
		$urls = apply_filters( 'wp_resource_hints', $urls, $relation_type );

		foreach ( $urls as $key => $url ) {
			$atts = array();

			if ( is_array( $url ) ) {
				if ( isset( $url['href'] ) ) {
					$atts = $url;
					$url  = $url['href'];
				} else {
					continue;
				}
			}

			$url = esc_url( $url, array( 'http', 'https' ) );

			if ( ! $url ) {
				continue;
			}

			if ( isset( $unique_urls[ $url ] ) ) {
				continue;
			}

			if ( in_array( $relation_type, array( 'preconnect', 'dns-prefetch' ), true ) ) {
				$parsed = wp_parse_url( $url );

				if ( empty( $parsed['host'] ) ) {
					continue;
				}

				if ( 'preconnect' === $relation_type && ! empty( $parsed['scheme'] ) ) {
					$url = $parsed['scheme'] . '://' . $parsed['host'];
				} else {
					// Use protocol-relative URLs for dns-prefetch or if scheme is missing.
					$url = '//' . $parsed['host'];
				}
			}

			$atts['rel']  = $relation_type;
			$atts['href'] = $url;

			$unique_urls[ $url ] = $atts;
		}

		foreach ( $unique_urls as $atts ) {
			$html = '';

			foreach ( $atts as $attr => $value ) {
				if ( ! is_scalar( $value )
					|| ( ! in_array( $attr, array( 'as', 'crossorigin', 'href', 'pr', 'rel', 'type' ), true ) && ! is_numeric( $attr ) )
				) {

					continue;
				}

				$value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );

				if ( ! is_string( $attr ) ) {
					$html .= " $value";
				} else {
					$html .= " $attr='$value'";
				}
			}

			$html = trim( $html );

			echo "<link $html />\n";
		}
	}
}


Top ↑

Changelog Changelog

Changelog
Version Description
4.6.0 Introduced.

Top ↑

User Contributed Notes User Contributed Notes

  1. Skip to note 1 content
    Contributed by rmdundon

    Sometimes, you may see this, removing the entire action in order to remove the reference to w.org:

    remove_action( 'wp_head', 'wp_resource_hints', 2 ); // removes dns-prefetch for w.org
    

    However, that eliminates the benefit of having dns-prefetch automatically added for external scripts.

    An alternative is just to remove the hard-coded reference, and keep dns-prefetch for performance gains:

    add_filter( 'wp_resource_hints', function( $urls ) {
    
        foreach ($urls as $key => $url) {
            // Remove dns-prefetch for w.org (we really don't need it)
            // See https://core.trac.wordpress.org/ticket/40426 for details 
            if ( 'https://s.w.org/images/core/emoji/13.0.0/svg/' === $url ) {
                unset( $urls[ $key ] );
            }
        }
    
        return $urls;
    } );
    

You must log in before being able to contribute a note or feedback.