fix: check srcset when hydrating to prevent needless requests

.changeset/six-teachers-divide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check srcset when hydrating to prevent needless requests

packages/svelte/src/compiler/compile/render_dom/wrappers/Element/Attribute.js

@@ -64,6 +64,9 @@ export default class AttributeWrapper extends BaseAttributeWrapper {
 	/** @type {boolean} */
 	is_src;
 
+	/** @type {boolean} */
+	is_srcset;
+
 	/** @type {boolean} */
 	is_select_value_attribute;
 
@@ -120,6 +123,9 @@ export default class AttributeWrapper extends BaseAttributeWrapper {
 		this.is_src =
 			this.name === 'src' &&
 			(!this.parent.node.namespace || this.parent.node.namespace === namespaces.html);
+		this.is_srcset =
+			this.name === 'srcset' &&
+			(!this.parent.node.namespace || this.parent.node.namespace === namespaces.html);
 		this.should_cache = should_cache(this);
 	}
 
@@ -164,6 +170,11 @@ export default class AttributeWrapper extends BaseAttributeWrapper {
 				b`if (!@src_url_equal(${element.var}.src, ${init})) ${method}(${element.var}, "${name}", ${this.last});`
 			);
 			updater = b`${method}(${element.var}, "${name}", ${should_cache ? this.last : value});`;
+		} else if (this.is_srcset) {
+			block.chunks.hydrate.push(
+				b`if (!@srcset_url_equal(${element.var}, ${init})) ${method}(${element.var}, "${name}", ${this.last});`
+			);
+			updater = b`${method}(${element.var}, "${name}", ${should_cache ? this.last : value});`;
 		} else if (property_name) {
 			block.chunks.hydrate.push(b`${element.var}.${property_name} = ${init};`);
 			updater = block.renderer.options.dev
@@ -403,7 +414,7 @@ Object.keys(attribute_lookup).forEach((name) => {
 
 /** @param {AttributeWrapper} attribute */
 function should_cache(attribute) {
-	return attribute.is_src || attribute.node.should_cache();
+	return attribute.is_src || attribute.is_srcset || attribute.node.should_cache();
 }
 const regex_contains_checked_or_group = /checked|group/;
 

packages/svelte/src/runtime/internal/utils.js

@@ -68,7 +68,11 @@ export function safe_not_equal(a, b) {
 
 let src_url_equal_anchor;
 
-/** @returns {boolean} */
+/**
+ * @param {string} element_src
+ * @param {string} url
+ * @returns {boolean}
+ */
 export function src_url_equal(element_src, url) {
 	if (!src_url_equal_anchor) {
 		src_url_equal_anchor = document.createElement('a');
@@ -77,6 +81,26 @@ export function src_url_equal(element_src, url) {
 	return element_src === src_url_equal_anchor.href;
 }
 
+/**
+ * @param {HTMLSourceElement | HTMLImageElement} element_srcset
+ * @param {string} srcset
+ * @returns {boolean}
+ */
+export function srcset_url_equal(element_srcset, srcset) {
+	/** @param {string} _srcset */
+	function split(_srcset) {
+		return _srcset.split(',').map((src) => src.trim().split(' ')[0]);
+	}
+
+	const element_urls = split(element_srcset.srcset);
+	const urls = split(srcset);
+
+	return (
+		urls.length === element_urls.length &&
+		urls.every((url, i) => src_url_equal(element_urls[i], url))
+	);
+}
+
 /** @returns {boolean} */
 export function not_equal(a, b) {
 	return a != a ? b == b : a !== b;