fix: add ability to pass functions in config

index.js

@@ -3,12 +3,10 @@ const { Transformer } = require('@parcel/plugin');
 const { default: SourceMap } = require('@parcel/source-map');
 const { relativeUrl } = require('@parcel/utils');
 const { compile, preprocess } = require('svelte/compiler.js');
+const { load, preSerialize, postDeserialize } = require('./loadConfig');
 
 Object.defineProperty(exports, '__esModule', { value: true });
 
-const CONFIG_FILES = ['.svelterc', 'svelte.config.js'];
-const CONFIG_PACKAGE_KEY = 'svelte';
-
 function generateName(input) {
   let name = path
     .basename(input)
@@ -40,51 +38,31 @@ async function handleError(sourceFileName, func) {
   }
 }
 
-async function getConfig(config) {
-  const packageKey = CONFIG_PACKAGE_KEY;
-  const data = await config.getConfig(CONFIG_FILES, { packageKey });
-
-  return data && data.contents ? data.contents : {};
-}
-
 exports.default = new Transformer({
-  async loadConfig({ config, options }) {
-    const customOptions = await getConfig(config);
-
-    if (customOptions.compiler) {
-      console.error(
-        'The "compiler" option in .svelterc is deprecated, use "compilerOptions" instead',
-      );
-      customOptions.compilerOptions =
-        customOptions.compilerOptions || customOptions.compiler;
-    }
-
-    const compilerOptions = {
-      css: false,
-      ...customOptions.compilerOptions,
+  loadConfig({ config, options, logger }) {
+    return load({ config, options, logger });
+  },
 
-      dev: options.mode !== 'production',
-    };
-    const preprocess = customOptions.preprocess;
+  preSerializeConfig({ config }) {
+    return preSerialize(config);
+  },
 
-    config.setResult({
-      compilerOptions,
-      preprocess,
-    });
+  postDeserializeConfig({ config, options }) {
+    return postDeserialize(config, options);
   },
 
   async transform({ asset, config, options }) {
     let code = await asset.getCode();
     const sourceFileName = relativeUrl(options.projectRoot, asset.filePath);
     const compilerOptions = {
-      ...config.compilerOptions,
+      ...config.raw.compilerOptions,
       filename: sourceFileName,
       name: generateName(sourceFileName),
     };
 
-    if (config.preprocess) {
+    if (config.hydrated.preprocess) {
       const preprocessed = await handleError(sourceFileName, () =>
-        preprocess(code, config.preprocess, compilerOptions),
+        preprocess(code, config.hydrated.preprocess, compilerOptions),
       );
       code = preprocessed.toString();
     }

loadConfig.js

@@ -0,0 +1,87 @@
+const path = require('path');
+
+Object.defineProperty(exports, '__esModule', { value: true });
+
+const CONFIG_FILES = ['.svelterc', 'svelte.config.js'];
+const CONFIG_PACKAGE_KEY = 'svelte';
+
+async function getConfigFile(config) {
+  const packageKey = CONFIG_PACKAGE_KEY;
+  const file = await config.getConfig(CONFIG_FILES, { packageKey });
+
+  return file || null;
+}
+
+async function configHydrator(configFile, config, options) {
+  config.setResult({
+    raw: configFile,
+    hydrated: {
+      preprocess: configFile.preprocess,
+    },
+  });
+}
+
+exports.load = async function load({ config, options, logger }) {
+  const configFile = await getConfigFile(config);
+  if (!configFile) return;
+
+  const { contents } = configFile;
+  const isDynamic = path.extname(configFile.filePath) === '.js';
+
+  if (typeof contents !== 'object') {
+    throw new Error('Svelte config should be an object.');
+  }
+
+  if (isDynamic) {
+    if (!contents.preprocess) {
+      logger.warn({
+        message:
+          'WARNING: Using a JavaScript Svelte config file means losing out on caching features of Parcel. Use a .svelterc(.json) file whenever possible.',
+      });
+    }
+
+    config.shouldInvalidateOnStartup();
+
+    if (contents.preprocess) {
+      config.shouldReload();
+    }
+  }
+
+  if (contents.compiler) {
+    logger.warn({
+      message:
+        'WARNING: The "compiler" option in .svelterc is deprecated, use "compilerOptions" instead',
+    });
+    contents.compilerOptions = contents.compilerOptions || contents.compiler;
+  }
+
+  return configHydrator(
+    {
+      compilerOptions: {
+        css: false,
+        ...contents.compilerOptions,
+
+        dev: options.mode !== 'production',
+      },
+      preprocess: contents.preprocess,
+    },
+    config,
+    options,
+  );
+};
+
+exports.preSerialize = function preSerialize(config) {
+  if (!config.result) return;
+
+  // Ensure we don't pass preprocess functions to the serializer
+  if (config.result.raw.preprocess) {
+    config.result.raw = {};
+  }
+
+  // This gets re-hydrated in Deserialize, so never store this
+  config.result.hydrated = {};
+};
+
+exports.postDeserialize = function postDeserialize(config, options) {
+  return configHydrator(config.result.raw, config, options);
+};