WIP

packages/svelte/package.json

@@ -142,6 +142,7 @@
     "acorn-typescript": "^1.4.13",
     "aria-query": "^5.3.1",
     "axobject-query": "^4.1.0",
+    "clsx": "^2.1.1",
     "esm-env": "^1.0.0",
     "esrap": "^1.2.2",
     "is-reference": "^3.0.2",

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -739,6 +739,8 @@ export function analyze_component(root, source, options) {
 
 					if (attribute.type !== 'Attribute') continue;
 					if (attribute.name.toLowerCase() !== 'class') continue;
+					// The dynamic class method appends the hash to the end of the class attribute on its own
+					if (attribute.metadata.is_dynamic_class) continue outer;
 
 					class_attribute = attribute;
 				}

packages/svelte/src/compiler/phases/2-analyze/visitors/Attribute.js

@@ -24,6 +24,12 @@ export function Attribute(node, context) {
 		}
 	}
 
+	// class={[...]} or class={{...}} or `class={x}` need clsx to resolve the classes
+	if (node.name === 'class' && !Array.isArray(node.value) && node.value !== true) {
+		mark_subtree_dynamic(context.path);
+		node.metadata.is_dynamic_class = true;
+	}
+
 	if (node.value !== true) {
 		for (const chunk of get_attribute_chunks(node.value)) {
 			if (chunk.type !== 'ExpressionTag') continue;

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -544,6 +544,10 @@ function build_element_attribute_update_assignment(element, node_id, attribute,
 	let update;
 
 	if (name === 'class') {
+		if (attribute.metadata.is_dynamic_class) {
+			value = b.call('$.clsx', value);
+		}
+
 		if (attribute.metadata.expression.has_state && has_call) {
 			// ensure we're not creating a separate template effect for this so that
 			// potential class directives are added to the same effect and therefore always apply
@@ -552,11 +556,13 @@ function build_element_attribute_update_assignment(element, node_id, attribute,
 			value = b.call('$.get', id);
 			has_call = false;
 		}
+
 		update = b.stmt(
 			b.call(
 				is_svg ? '$.set_svg_class' : is_mathml ? '$.set_mathml_class' : '$.set_class',
 				node_id,
-				value
+				value,
+				attribute.metadata.is_dynamic_class ? b.literal(context.state.analysis.css.hash) : undefined
 			)
 		);
 	} else if (name === 'value') {

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/element.js

@@ -85,10 +85,25 @@ export function build_element_attributes(node, context) {
 			} else {
 				if (attribute.name === 'class') {
 					class_index = attributes.length;
-				} else if (attribute.name === 'style') {
-					style_index = attributes.length;
+					if (attribute.metadata.is_dynamic_class) {
+						attributes.push({
+							...attribute,
+							value: {
+								.../** @type {AST.ExpressionTag} */ (attribute.value),
+								expression: b.call(
+									'$.clsx',
+									/** @type {AST.ExpressionTag} */ (attribute.value).expression,
+									b.literal(context.state.analysis.css.hash)
+								)
+							}
+						});
+					}
+				} else {
+					if (attribute.name === 'style') {
+						style_index = attributes.length;
+					}
+					attributes.push(attribute);
 				}
-				attributes.push(attribute);
 			}
 		} else if (attribute.type === 'BindDirective') {
 			if (attribute.name === 'value' && node.name === 'select') continue;

packages/svelte/src/compiler/phases/nodes.js

@@ -46,7 +46,8 @@ export function create_attribute(name, start, end, value) {
 		parent: null,
 		metadata: {
 			expression: create_expression_metadata(),
-			delegated: null
+			delegated: null,
+			is_dynamic_class: false
 		}
 	};
 }

packages/svelte/src/compiler/types/template.d.ts

@@ -452,6 +452,8 @@ export namespace AST {
 			expression: ExpressionMetadata;
 			/** May be set if this is an event attribute */
 			delegated: null | DelegatedEvent;
+			/** May be `true` if this is a `class` attribute that needs `clsx` */
+			is_dynamic_class: boolean;
 		};
 	}
 

packages/svelte/src/internal/client/dom/elements/class.js

@@ -61,12 +61,13 @@ export function set_mathml_class(dom, value) {
 /**
  * @param {HTMLElement} dom
  * @param {string} value
+ * @param {string} [hash]
  * @returns {void}
  */
-export function set_class(dom, value) {
+export function set_class(dom, value, hash) {
 	// @ts-expect-error need to add __className to patched prototype
 	var prev_class_name = dom.__className;
-	var next_class_name = to_class(value);
+	var next_class_name = to_class(value) + (hash != null ? ' ' + hash : '');
 
 	if (hydrating && dom.className === next_class_name) {
 		// In case of hydration don't reset the class as it's already correct.

packages/svelte/src/internal/client/index.js

@@ -1,3 +1,4 @@
+export { clsx } from 'clsx';
 export { FILENAME, HMR, NAMESPACE_SVG } from '../../constants.js';
 export { cleanup_styles } from './dev/css.js';
 export { add_locations } from './dev/elements.js';

packages/svelte/src/internal/server/index.js

@@ -1,6 +1,7 @@
 /** @import { ComponentType, SvelteComponent } from 'svelte' */
 /** @import { Component, Payload, RenderOutput } from '#server' */
 /** @import { Store } from '#shared' */
+export { clsx } from 'clsx';
 export { FILENAME, HMR } from '../../constants.js';
 import { is_promise, noop } from '../shared/utils.js';
 import { subscribe_to_store } from '../../store/utils.js';

pnpm-lock.yaml

@@ -80,6 +80,9 @@ importers:
       axobject-query:
         specifier: ^4.1.0
         version: 4.1.0
+      clsx:
+        specifier: ^2.1.1
+        version: 2.1.1
       esm-env:
         specifier: ^1.0.0
         version: 1.0.0
@@ -1211,6 +1214,10 @@ packages:
     resolution: {integrity: sha512-NIxF55hv4nSqQswkAeiOi1r83xy8JldOFDTWiug55KBu9Jnblncd2U6ViHmYgHf01TPZS77NJBhBMKdWj9HQMQ==}
     engines: {node: '>=8'}
 
+  clsx@2.1.1:
+    resolution: {integrity: sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA==}
+    engines: {node: '>=6'}
+
   codemirror@6.0.1:
     resolution: {integrity: sha512-J8j+nZ+CdWmIeFIGXEFbFPtpiYacFMDR8GlHK3IyHQJMCaVRfGx9NT+Hxivv1ckLWPvNdZqndbr/7lVhrf/Svg==}
 
@@ -3950,6 +3957,8 @@ snapshots:
 
   ci-info@3.9.0: {}
 
+  clsx@2.1.1: {}
+
   codemirror@6.0.1(@lezer/common@1.2.1):
     dependencies:
       '@codemirror/autocomplete': 6.18.0(@codemirror/language@6.10.2)(@codemirror/state@6.4.1)(@codemirror/view@6.33.0)(@lezer/common@1.2.1)