extends image preloading to accept imagesrcset and imagesizes to handle cases where srcset and sizes are used over src. Normally we key off an href however in the case of responsive images it is possible that no src attribute is desired. Right now this happens most prominently with Safari which does understand srcset for the img tag but not for the link preload tag. Because of this it can be detrimental to preload with a fallback src since Safari will end up downloading the wrong image 100% of the time.

The solution to this is to allow the user to omit the href if they provide imagesrcset (and optionally) imagesizes. Effectively the key for image preloads will become the union of src (href), imagesrcset, and imagesizes.

Author: gnoff

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -105,7 +105,6 @@ import {
 } from 'react-reconciler/src/ReactWorkTags';
 import {listenToAllSupportedEvents} from '../events/DOMPluginEventSystem';
 import {
-  validatePreloadArguments,
   validatePreinitArguments,
   validateLinkPropsForStyleResource,
   getValueDescriptorExpectingObjectForWarning,
@@ -2016,7 +2015,7 @@ type ScriptProps = {
 
 type PreloadProps = {
   rel: 'preload',
-  href: string,
+  href: ?string,
   [string]: mixed,
 };
 
@@ -2167,21 +2166,70 @@ function preload(href: string, options: PreloadOptions) {
     return;
   }
   if (__DEV__) {
-    validatePreloadArguments(href, options);
+    // TODO move this to ReactDOMFloat and expose a stricter function interface or possibly
+    // typed functions (preloadImage, preloadStyle, ...)
+    let encountered = '';
+    if (typeof href !== 'string' || !href) {
+      encountered += `The \`href\` argument encountered was ${getValueDescriptorExpectingObjectForWarning(
+        href,
+      )}.`;
+    }
+    if (options == null || typeof options !== 'object') {
+      encountered += `The \`options\` argument encountered was ${getValueDescriptorExpectingObjectForWarning(
+        options,
+      )}.`;
+    } else if (typeof options.as !== 'string' || !options.as) {
+      encountered += `The \`as\` option encountered was ${getValueDescriptorExpectingObjectForWarning(
+        options.as,
+      )}.`;
+    }
+    if (encountered) {
+      console.error(
+        'ReactDOM.preload(): Expected two arguments, a non-empty `href` string and an `options` object with an `as` property valid for a `<link rel="preload" as="..." />` tag. %s',
+        encountered,
+      );
+    }
   }
   const ownerDocument = getDocumentForImperativeFloatMethods();
   if (
     typeof href === 'string' &&
     href &&
     typeof options === 'object' &&
     options !== null &&
+    typeof options.as === 'string' &&
+    options.as &&
     ownerDocument
   ) {
     const as = options.as;
-    const limitedEscapedHref =
-      escapeSelectorAttributeValueInsideDoubleQuotes(href);
-    const preloadSelector = `link[rel="preload"][as="${as}"][href="${limitedEscapedHref}"]`;
-
+    let preloadSelector = `link[rel="preload"][as="${escapeSelectorAttributeValueInsideDoubleQuotes(
+      as,
+    )}"]`;
+    if (as === 'image') {
+      const {imageSrcSet, imageSizes} = options;
+      if (typeof imageSrcSet === 'string' && imageSrcSet !== '') {
+        preloadSelector += `[imagesrcset="${escapeSelectorAttributeValueInsideDoubleQuotes(
+          imageSrcSet,
+        )}"]`;
+        if (typeof imageSizes === 'string') {
+          preloadSelector += `[imagesizes="${escapeSelectorAttributeValueInsideDoubleQuotes(
+            imageSizes,
+          )}"]`;
+        }
+      } else {
+        preloadSelector += `[href="${escapeSelectorAttributeValueInsideDoubleQuotes(
+          href,
+        )}"]`;
+      }
+    } else {
+      if (!href) {
+        // This call has no href and its type does not specify an alternate preloadabl
+        // resources so we noop
+        return;
+      }
+      preloadSelector += `[href="${escapeSelectorAttributeValueInsideDoubleQuotes(
+        href,
+      )}"]`;
+    }
     // Some preloads are keyed under their selector. This happens when the preload is for
     // an arbitrary type. Other preloads are keyed under the resource key they represent a preload for.
     // Here we figure out which key to use to determine if we have a preload already.
@@ -2227,14 +2275,20 @@ function preloadPropsFromPreloadOptions(
   options: PreloadOptions,
 ): PreloadProps {
   return {
-    href,
     rel: 'preload',
     as,
+    // There is a bug in Safari where imageSrcSet is not respected on preload links
+    // so we omit the href here if we have imageSrcSet b/c safari will load the wrong image.
+    // This harms older browers that do not support imageSrcSet by making their preloads not work
+    // but this population is shrinking fast and is already small so we accept this tradeoff.
+    href: as === 'image' && options.imageSrcSet ? undefined : href,
     crossOrigin: as === 'font' ? '' : options.crossOrigin,
     integrity: options.integrity,
     type: options.type,
     nonce: options.nonce,
     fetchPriority: options.fetchPriority,
+    imageSrcSet: options.imageSrcSet,
+    imageSizes: options.imageSizes,
   };
 }
 

packages/react-dom-bindings/src/server/ReactFizzConfigDOM.js

@@ -4819,12 +4819,12 @@ type PreconnectResource = TResource<'preconnect', null>;
 type PreloadAsProps = {
   rel: 'preload',
   as: string,
-  href: string,
+  href: ?string,
   [string]: mixed,
 };
 type PreloadModuleProps = {
   rel: 'modulepreload',
-  href: string,
+  href: ?string,
   [string]: mixed,
 };
 type PreloadProps = PreloadAsProps | PreloadModuleProps;
@@ -5063,20 +5063,25 @@ export function preload(href: string, options: PreloadOptions) {
   }
   const resources = getResources(request);
   if (__DEV__) {
+    let encountered = '';
     if (typeof href !== 'string' || !href) {
+      encountered += ` The \`href\` argument encountered was ${getValueDescriptorExpectingObjectForWarning(
+        href,
+      )}.`;
+    }
+    if (options == null || typeof options !== 'object') {
+      encountered += ` The \`options\` argument encountered was ${getValueDescriptorExpectingObjectForWarning(
+        options,
+      )}.`;
+    } else if (typeof options.as !== 'string' || !options.as) {
+      encountered += ` The \`as\` option encountered was ${getValueDescriptorExpectingObjectForWarning(
+        options.as,
+      )}.`;
+    }
+    if (encountered) {
       console.error(
-        'ReactDOM.preload(): Expected the `href` argument (first) to be a non-empty string but encountered %s instead.',
-        getValueDescriptorExpectingObjectForWarning(href),
-      );
-    } else if (options == null || typeof options !== 'object') {
-      console.error(
-        'ReactDOM.preload(): Expected the `options` argument (second) to be an object with an `as` property describing the type of resource to be preloaded but encountered %s instead.',
-        getValueDescriptorExpectingEnumForWarning(options),
-      );
-    } else if (typeof options.as !== 'string') {
-      console.error(
-        'ReactDOM.preload(): Expected the `as` property in the `options` argument (second) to contain a string value describing the type of resource to be preloaded but encountered %s instead. Values that are valid in for the `as` attribute of a `<link rel="preload" as="..." />` tag are valid here.',
-        getValueDescriptorExpectingEnumForWarning(options.as),
+        'ReactDOM.preload(): Expected two arguments, a non-empty `href` string and an `options` object with an `as` property valid for a `<link rel="preload" as="..." />` tag.%s',
+        encountered,
       );
     }
   }
@@ -5085,10 +5090,34 @@ export function preload(href: string, options: PreloadOptions) {
     href &&
     typeof options === 'object' &&
     options !== null &&
-    typeof options.as === 'string'
+    typeof options.as === 'string' &&
+    options.as
   ) {
     const as = options.as;
-    const key = getResourceKey(as, href);
+    let key: string;
+    if (as === 'image') {
+      // For image preloads the key contains either the imageSrcSet + imageSizes or the href but not
+      // both. This is to prevent identical calls with the same srcSet and sizes to be duplicated
+      // by varying the href. this is an edge case but it is the most correct behavior.
+      const {imageSrcSet, imageSizes} = options;
+      let uniquePart = '';
+      if (typeof imageSrcSet === 'string' && imageSrcSet !== '') {
+        uniquePart += '[' + imageSrcSet + ']';
+        if (typeof imageSizes === 'string') {
+          uniquePart += '[' + imageSizes + ']';
+        }
+      } else {
+        uniquePart += '[][]' + href;
+      }
+      key = getResourceKey(as, uniquePart);
+    } else {
+      if (!href) {
+        // This call has no href and its type does not specify an alternate preloadabl
+        // resources so we noop
+        return;
+      }
+      key = getResourceKey(as, href);
+    }
     let resource = resources.preloadsMap.get(key);
     if (__DEV__) {
       const devResource = getAsResourceDEV(resource);
@@ -5528,12 +5557,18 @@ function preloadPropsFromPreloadOptions(
   return {
     rel: 'preload',
     as,
-    href,
+    // There is a bug in Safari where imageSrcSet is not respected on preload links
+    // so we omit the href here if we have imageSrcSet b/c safari will load the wrong image.
+    // This harms older browers that do not support imageSrcSet by making their preloads not work
+    // but this population is shrinking fast and is already small so we accept this tradeoff.
+    href: as === 'image' && options.imageSrcSet ? undefined : href,
     crossOrigin: as === 'font' ? '' : options.crossOrigin,
     integrity: options.integrity,
     type: options.type,
     nonce: options.nonce,
     fetchPriority: options.fetchPriority,
+    imageSrcSet: options.imageSrcSet,
+    imageSizes: options.imageSizes,
   };
 }
 

packages/react-dom-bindings/src/shared/ReactDOMResourceValidation.js

@@ -62,60 +62,6 @@ function propNamesListJoin(
   }
 }
 
-export function validatePreloadArguments(href: mixed, options: mixed) {
-  if (__DEV__) {
-    if (!href || typeof href !== 'string') {
-      const typeOfArg = getValueDescriptorExpectingObjectForWarning(href);
-      console.error(
-        'ReactDOM.preload() expected the first argument to be a string representing an href but found %s instead.',
-        typeOfArg,
-      );
-    } else if (typeof options !== 'object' || options === null) {
-      const typeOfArg = getValueDescriptorExpectingObjectForWarning(options);
-      console.error(
-        'ReactDOM.preload() expected the second argument to be an options argument containing at least an "as" property' +
-          ' specifying the Resource type. It found %s instead. The href for the preload call where this warning originated is "%s".',
-        typeOfArg,
-        href,
-      );
-    } else {
-      const as = options.as;
-      switch (as) {
-        // Font specific validation of options
-        case 'font': {
-          if (options.crossOrigin === 'use-credentials') {
-            console.error(
-              'ReactDOM.preload() was called with an "as" type of "font" and with a "crossOrigin" option of "use-credentials".' +
-                ' Fonts preloading must use crossOrigin "anonymous" to be functional. Please update your font preload to omit' +
-                ' the crossOrigin option or change it to any other value than "use-credentials" (Browsers default all other values' +
-                ' to anonymous mode). The href for the preload call where this warning originated is "%s"',
-              href,
-            );
-          }
-          break;
-        }
-        case 'script':
-        case 'style': {
-          break;
-        }
-
-        // We have an invalid as type and need to warn
-        default: {
-          const typeOfAs = getValueDescriptorExpectingEnumForWarning(as);
-          console.error(
-            'ReactDOM.preload() expected a valid "as" type in the options (second) argument but found %s instead.' +
-              ' Please use one of the following valid values instead: %s. The href for the preload call where this' +
-              ' warning originated is "%s".',
-            typeOfAs,
-            '"style", "font", or "script"',
-            href,
-          );
-        }
-      }
-    }
-  }
-}
-
 export function validatePreinitArguments(href: mixed, options: mixed) {
   if (__DEV__) {
     if (!href || typeof href !== 'string') {