astro/packages/integrations/svelte/src/index.ts

import type { Options } from '@sveltejs/vite-plugin-svelte';
import { svelte } from '@sveltejs/vite-plugin-svelte';
import type { AstroIntegration, AstroRenderer } from 'astro';
import preprocess from 'svelte-preprocess';

function getRenderer(): AstroRenderer {
	return {
		name: '@astrojs/svelte',
		clientEntrypoint: '@astrojs/svelte/client.js',
		serverEntrypoint: '@astrojs/svelte/server.js',
	};
}

function getViteConfiguration(isDev: boolean, options?: Options | OptionsCallback) {
	const defaultOptions = {
		emitCss: true,
		compilerOptions: { dev: isDev, hydratable: true },
		preprocess: [
			preprocess({
				less: true,
				sass: { renderSync: true },
				scss: { renderSync: true },
				stylus: true,
				typescript: true,
			}),
		],
	};

	let resolvedOptions: Partial<Options>;

	if (!options) {
		resolvedOptions = defaultOptions;
	} else if (typeof options === 'function') {
		resolvedOptions = options(defaultOptions);
	} else {
		resolvedOptions = {
			...options,
			...defaultOptions,
			compilerOptions: {
				...options.compilerOptions,
				// Always use dev and hydratable from defaults
				...defaultOptions.compilerOptions,
			},
			// Ignore default preprocessor if the user provided their own
			preprocess: options.preprocess ?? defaultOptions.preprocess,
		};
	}

	return {
		optimizeDeps: {
			include: ['@astrojs/svelte/client.js', 'svelte', 'svelte/internal'],
			exclude: ['@astrojs/svelte/server.js'],
		},
		plugins: [svelte(resolvedOptions)],
	};
}

type OptionsCallback = (defaultOptions: Options) => Options;
export default function (options?: Options | OptionsCallback): AstroIntegration {
	return {
		name: '@astrojs/svelte',
		hooks: {
			// Anything that gets returned here is merged into Astro Config
			'astro:config:setup': ({ command, updateConfig, addRenderer }) => {
				addRenderer(getRenderer());
				updateConfig({ vite: getViteConfiguration(command === 'dev', options) });
			},
		},
	};
}