feat!: vite 6 support

packages/e2e-tests/kit-node/__tests__/kit.spec.ts

@@ -14,6 +14,7 @@ import {
 	readFileContent
 } from '~utils';
 
+import { version as viteVersion } from 'vite';
 import glob from 'tiny-glob';
 import path from 'node:path';
 import { describe, expect, it } from 'vitest';
@@ -347,11 +348,19 @@ describe('kit-node', () => {
 					['svelte', 'browser', 'module', 'jsnext:main', 'jsnext'],
 					`resolve.mainFields in ${filename}`
 				);
-				expectArrayEqual(
-					config.resolve.conditions,
-					['svelte'],
-					`resolve.conditions in ${filename}`
-				);
+				if (viteVersion.startsWith('6.')) {
+					expectArrayEqual(
+						config.resolve.conditions,
+						['browser', 'development|production', 'module', 'svelte'],
+						`resolve.conditions in ${filename}`
+					);
+				} else {
+					expectArrayEqual(
+						config.resolve.conditions,
+						['svelte'],
+						`resolve.conditions in ${filename}`
+					);
+				}
 			}
 		});
 	});

packages/e2e-tests/vite-ssr-esm/vite.config.js

@@ -14,6 +14,11 @@ export default defineConfig(({ command, mode }) => {
 				}
 			}
 		},
+		// TODO: investigate the condition issue. it's the same thing dominik and ben found.
+		// idk why it's only happening after https://github.com/vitejs/vite/pull/18395
+		ssr: {
+			noExternal: ['esm-env']
+		},
 		server: {
 			watch: {
 				// During tests we edit the files too fast and sometimes chokidar

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

@@ -64,6 +64,35 @@ export function svelte(inlineOptions) {
 				return extraViteConfig;
 			},
 
+			// @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) {
+					// 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 = [...defaultServerConditions];
+					}
+				}
+				config.resolve.conditions.push('svelte');
+			},
+
 			async configResolved(config) {
 				options = resolveOptions(options, config, cache);
 				patchResolvedViteConfig(config, options);

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

@@ -30,6 +30,7 @@ import {
 
 import { isCommonDepWithoutSvelteField } from './dependencies.js';
 import { VitePluginSvelteStats } from './vite-plugin-svelte-stats.js';
+import { isVite6 } from './vite-version.js';
 
 const allowedPluginOptions = new Set([
 	'include',
@@ -333,21 +334,25 @@ 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],
-			conditions: [...SVELTE_EXPORT_CONDITIONS]
+			// 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
 		// see https://github.com/sveltejs/vite-plugin-svelte/issues/60

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

@@ -0,0 +1,6 @@
+import { version } from 'vite';
+
+/**
+ * @type {boolean}
+ */
+export const isVite6 = version.startsWith('6.');

packages/vite-plugin-svelte/types/index.d.ts.map

@@ -22,5 +22,5 @@
 		null,
 		null
 	],
-	"mappings": ";;;;aAIYA,OAAOA;;;;;;;;;;;;;kBAaFC,aAAaA;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;kBAgGbC,YAAYA;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;kBAkFZC,qBAAqBA;;;;;;;;;;;;;iBC1KtBC,MAAMA;iBCTNC,cAAcA;iBCgBRC,gBAAgBA"
+	"mappings": ";;;;aAIYA,OAAOA;;;;;;;;;;;;;kBAaFC,aAAaA;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;kBAgGbC,YAAYA;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;kBAkFZC,qBAAqBA;;;;;;;;;;;;;iBCtKtBC,MAAMA;iBCbNC,cAAcA;iBCgBRC,gBAAgBA"
 }