feat: Customization of serialization for languageOptions (#19760)

* feat: Customization serialization for languageOptions

fixes #19759

* Update docs and tests

* Check result of toJSON is not a function and throw error

* Allow toJSON() on languageOptions

* Reduce duplicated code

* Prefer toJSON over meta

Author: nzakas

docs/src/extend/languages.md

@@ -88,7 +88,7 @@ The following optional members allow you to customize how ESLint interacts with
 - `visitorKeys` - visitor keys that are specific to the AST or CST. This is used to optimize traversal of the AST or CST inside of ESLint. While not required, it is strongly recommended, especially for AST or CST formats that deviate significantly from ESTree format.
 - `defaultLanguageOptions` - default `languageOptions` when the language is used. User-specified `languageOptions` are merged with this object when calculating the config for the file being linted.
 - `matchesSelectorClass(className, node, ancestry)` - allows you to specify selector classes, such as `:expression`, that match more than one node. This method is called whenever an [esquery](https://github.com/estools/esquery) selector contains a `:` followed by an identifier.
-- `normalizeLanguageOptions(languageOptions)` - takes a validated language options object and normalizes its values. This is helpful for backwards compatibility when language options properties change.
+- `normalizeLanguageOptions(languageOptions)` - takes a validated language options object and normalizes its values. This is helpful for backwards compatibility when language options properties change and also to add custom serialization with a `toJSON()` method.
 
 See [`JSONLanguage`](https://github.com/eslint/json/blob/main/src/languages/json-language.js) as an example of a basic `Language` class.
 

lib/config/config.js

@@ -265,6 +265,27 @@ function getObjectId(object) {
 	return name;
 }
 
+/**
+ * Asserts that a value is not a function.
+ * @param {any} value The value to check.
+ * @param {string} key The key of the value in the object.
+ * @param {string} objectKey The key of the object being checked.
+ * @returns {void}
+ * @throws {TypeError} If the value is a function.
+ */
+function assertNotFunction(value, key, objectKey) {
+	if (typeof value === "function") {
+		const error = new TypeError(
+			`Cannot serialize key "${key}" in "${objectKey}": Function values are not supported.`,
+		);
+
+		error.messageTemplate = "config-serialize-function";
+		error.messageData = { key, objectKey };
+
+		throw error;
+	}
+}
+
 /**
  * Converts a languageOptions object to a JSON representation.
  * @param {Record<string, any>} languageOptions The options to create a JSON
@@ -274,31 +295,33 @@ function getObjectId(object) {
  * @throws {TypeError} If a function is found in the languageOptions.
  */
 function languageOptionsToJSON(languageOptions, objectKey = "languageOptions") {
+	if (typeof languageOptions.toJSON === "function") {
+		const result = languageOptions.toJSON();
+
+		assertNotFunction(result, "toJSON", objectKey);
+
+		return result;
+	}
+
 	const result = {};
 
 	for (const [key, value] of Object.entries(languageOptions)) {
 		if (value) {
 			if (typeof value === "object") {
 				const name = getObjectId(value);
 
-				if (name && hasMethod(value)) {
+				if (typeof value.toJSON === "function") {
+					result[key] = value.toJSON();
+					assertNotFunction(result[key], key, objectKey);
+				} else if (name && hasMethod(value)) {
 					result[key] = name;
 				} else {
 					result[key] = languageOptionsToJSON(value, key);
 				}
 				continue;
 			}
 
-			if (typeof value === "function") {
-				const error = new TypeError(
-					`Cannot serialize key "${key}" in ${objectKey}: Function values are not supported.`,
-				);
-
-				error.messageTemplate = "config-serialize-function";
-				error.messageData = { key, objectKey };
-
-				throw error;
-			}
+			assertNotFunction(value, key, objectKey);
 		}
 
 		result[key] = value;