implement config.kit.alias (#4964)

* implement basic alias config (closes #4734)

* revise comments as benmccann says

* document config.kit.alias

* fix config default test

* Update documentation/docs/14-configuration.md

Co-authored-by: Rich Harris <hello@rich-harris.dev>

.changeset/real-mice-argue.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Add `config.kit.alias`

documentation/docs/14-configuration.md

@@ -16,6 +16,7 @@ const config = {
 
 	kit: {
 		adapter: undefined,
+		alias: {},
 		appDir: '_app',
 		browser: {
 			hydrate: true,
@@ -91,6 +92,27 @@ export default config;
 
 Required when running `svelte-kit build` and determines how the output is converted for different platforms. See [Adapters](/docs/adapters).
 
+### alias
+
+An object containing zero or more aliases used to replace values in `import` statements. These aliases are automatically passed to Vite and TypeScript.
+
+For example, you can add aliases to a `components` and `utils` folder:
+
+```js
+/// file: svelte.config.js
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	kit: {
+		alias: {
+			$components: 'src/components',
+			$utils: 'src/utils'
+		}
+	}
+};
+```
+
+> The built-in `$lib` alias is controlled by `config.kit.files.lib` as it is used for packaging.
+
 ### appDir
 
 The directory relative to `paths.assets` where the built JS and CSS (and imported assets) are served from. (The filenames therein contain content-based hashes, meaning they can be cached indefinitely). Must not start or end with `/`.

packages/kit/src/core/config/index.spec.js

@@ -12,6 +12,7 @@ const get_defaults = (prefix = '') => ({
 	extensions: ['.svelte'],
 	kit: {
 		adapter: null,
+		alias: {},
 		amp: undefined,
 		appDir: '_app',
 		browser: {

packages/kit/src/core/config/options.js

@@ -39,6 +39,18 @@ const options = object(
 				return input;
 			}),
 
+			alias: validate({}, (input, keypath) => {
+				if (typeof input !== 'object') {
+					throw new Error(`${keypath} should be an object`);
+				}
+
+				for (const key in input) {
+					assert_string(input[key], `${keypath}.${key}`);
+				}
+
+				return input;
+			}),
+
 			// TODO: remove this for the 1.0 release
 			amp: error(
 				(keypath) =>

packages/kit/src/core/sync/write_tsconfig.js

@@ -34,19 +34,27 @@ export function write_tsconfig(config, cwd = process.cwd()) {
 		include.push(config_relative(`${dir}/**/*.svelte`));
 	});
 
+	/** @type {Record<string, string[]>} */
+	const paths = {};
+	const alias = {
+		$lib: project_relative(config.kit.files.lib),
+		...config.kit.alias
+	};
+	for (const [key, value] of Object.entries(alias)) {
+		if (fs.existsSync(project_relative(value))) {
+			paths[key] = [project_relative(value)];
+			paths[key + '/*'] = [project_relative(value) + '/*'];
+		}
+	}
+
 	write_if_changed(
 		out,
 		JSON.stringify(
 			{
 				compilerOptions: {
 					// generated options
 					baseUrl: config_relative('.'),
-					paths: fs.existsSync(config.kit.files.lib)
-						? {
-								$lib: [project_relative(config.kit.files.lib)],
-								'$lib/*': [project_relative(config.kit.files.lib + '/*')]
-						  }
-						: {},
+					paths,
 					rootDirs: [config_relative('.'), './types'],
 
 					// essential options

packages/kit/src/core/utils.js

@@ -80,11 +80,19 @@ export function get_mime_lookup(manifest_data) {
 
 /** @param {import('types').ValidatedConfig} config */
 export function get_aliases(config) {
+	/** @type {Record<string, string>} */
 	const alias = {
 		__GENERATED__: path.posix.join(config.kit.outDir, 'generated'),
 		$app: `${get_runtime_path(config)}/app`,
+
+		// For now, we handle `$lib` specially here rather than make it a default value for
+		// `config.kit.alias` since it has special meaning for packaging, etc.
 		$lib: config.kit.files.lib
 	};
 
+	for (const [key, value] of Object.entries(config.kit.alias)) {
+		alias[key] = path.resolve(value);
+	}
+
 	return alias;
 }

packages/kit/types/index.d.ts

@@ -93,6 +93,7 @@ export interface Config {
 	extensions?: string[];
 	kit?: {
 		adapter?: Adapter;
+		alias?: Record<string, string>;
 		appDir?: string;
 		browser?: {
 			hydrate?: boolean;