feat!: vite 6 support

packages/vite-plugin-svelte/src/index.js

@@ -18,10 +18,6 @@ import { saveSvelteMetadata } from './utils/optimizer.js';
 import { VitePluginSvelteCache } from './utils/vite-plugin-svelte-cache.js';
 import { loadRaw } from './utils/load-raw.js';
 import * as svelteCompiler from 'svelte/compiler';
-import {
-	VITE_CLIENT_RESOLVE_CONDITIONS,
-	VITE_SERVER_RESOLVE_CONDITIONS
-} from './utils/constants.js';
 
 /**
  * @param {Partial<import('./public.d.ts').Options>} [inlineOptions]
@@ -67,14 +63,31 @@ export function svelte(inlineOptions) {
 				log.debug('additional vite config', extraViteConfig, 'config');
 				return extraViteConfig;
 			},
-			// @ts-ignore Allow exist in vite 6
-			configEnvironment(name, config) {
+
+			// @ts-ignore This hook only works in Vite 6
+			async configEnvironment(name, config, opts) {
 				config.resolve ??= {};
+
+				// Emulate Vite default fallback for `resolve.mainFields` if not set
+				if (config.resolve.mainFields == null) {
+					// These exports only exist in Vite 6
+					const { defaultClientMainFields, defaultServerMainFields } = await import('vite');
+					if (name === 'client' || opts.isSsrTargetWebworker) {
+						config.resolve.mainFields = [...defaultClientMainFields];
+					} else {
+						config.resolve.mainFields = [...defaultServerMainFields];
+					}
+				}
+				config.resolve.mainFields.unshift('svelte');
+
+				// Emulate Vite default fallback for `resolve.conditions` if not set
 				if (config.resolve.conditions == null) {
-					if (name === 'client') {
-						config.resolve.conditions = [...VITE_CLIENT_RESOLVE_CONDITIONS];
+					// These exports only exist in Vite 6
+					const { defaultClientConditions, defaultServerConditions } = await import('vite');
+					if (name === 'client' || opts.isSsrTargetWebworker) {
+						config.resolve.conditions = [...defaultClientConditions];
 					} else {
-						config.resolve.conditions = [...VITE_SERVER_RESOLVE_CONDITIONS];
+						config.resolve.conditions = [...defaultServerConditions];
 					}
 				}
 				config.resolve.conditions.push('svelte');

packages/vite-plugin-svelte/src/utils/constants.js

@@ -2,11 +2,6 @@ import { createRequire } from 'node:module';
 
 export const VITE_RESOLVE_MAIN_FIELDS = ['browser', 'module', 'jsnext:main', 'jsnext'];
 
-// These two are required for Vite 6 only, as specifying conditions will remove the default ones,
-// like the `mainFields` option. Vite 6 is working on exposing these which we can use later.
-export const VITE_CLIENT_RESOLVE_CONDITIONS = ['module', 'browser', 'development|production'];
-export const VITE_SERVER_RESOLVE_CONDITIONS = ['module', 'node', 'development|production'];
-
 export const SVELTE_RESOLVE_MAIN_FIELDS = ['svelte'];
 
 export const SVELTE_IMPORTS = Object.entries(

packages/vite-plugin-svelte/src/utils/options.js

@@ -334,22 +334,24 @@ function resolveViteRoot(viteConfig) {
  * @returns {Promise<Partial<import('vite').UserConfig>>}
  */
 export async function buildExtraViteConfig(options, config) {
-	// make sure we only readd vite default mainFields when no other plugin has changed the config already
-	// see https://github.com/sveltejs/vite-plugin-svelte/issues/581
-	if (!config.resolve) {
-		config.resolve = {};
+	// `resolve.mainFields` override the defaults if set, but we want to extend it, so we directly mutate here.
+	// We only do so for Vite 5 and below, as in Vite 6, `resolve.mainFields` only apply to the client env,
+	// so we use the `configEnvironment` hook to set it up instead.
+	if (!isVite6) {
+		config.resolve ??= {};
+		config.resolve.mainFields = [
+			...SVELTE_RESOLVE_MAIN_FIELDS,
+			...(config.resolve.mainFields ?? VITE_RESOLVE_MAIN_FIELDS)
+		];
 	}
-	config.resolve.mainFields = [
-		...SVELTE_RESOLVE_MAIN_FIELDS,
-		...(config.resolve.mainFields ?? VITE_RESOLVE_MAIN_FIELDS)
-	];
 
 	/** @type {Partial<import('vite').UserConfig>} */
 	const extraViteConfig = {
 		resolve: {
 			dedupe: [...SVELTE_IMPORTS],
-			// In Vite 6, we need to provide the default conditions too as it now replaces the default,
-			// instead of extending it. We set undefined here and extend it in the `configEnvironment` hook instead.
+			// In Vite 6, conditions now override the defaults instead of extending. `resolve.conditions`
+			// also only apply to the client env, so we use the `configEnvironment` hook to set it up
+			// instead, so here we set to `undefined` to skip it.
 			conditions: isVite6 ? undefined : [...SVELTE_EXPORT_CONDITIONS]
 		}
 		// this option is still awaiting a PR in vite to be supported