feat: Add allowTrailingCommas option for JSONC

README.md

@@ -185,6 +185,35 @@ In JSONC and JSON5 files, you can also use [rule configurations comments](https:
 
 Both line and block comments can be used for all kinds of configuration comments.
 
+## Allowing trailing commas in JSONC
+
+The Microsoft implementation of JSONC optionally allows for trailing commas in objects and arrays (files like `tsconfig.json` have this option enabled by default in Visual Studio Code). To enable trailing commas in JSONC files, use the `allowTrailingCommas` language option, as in this example:
+
+```js
+import json from "@eslint/json";
+
+export default [
+	// lint JSONC files
+	{
+		files: ["**/*.jsonc"],
+		language: "json/jsonc",
+		...json.configs.recommended,
+	},
+
+	// lint JSONC files and allow trailing commas
+	{
+		files: ["**/tsconfig.json", ".vscode/*.json"],
+		language: "json/jsonc",
+		languageOptions: {
+			allowTrailingCommas: true
+		},
+		...json.configs.recommended,
+	},
+];
+```
+
+**Note:** The `allowTrailingCommas` option is only valid for the `json/jsonc` language.
+
 ## Frequently Asked Questions
 
 ### How does this relate to `eslint-plugin-json` and `eslint-plugin-jsonc`?

package.json

@@ -63,7 +63,7 @@
   "license": "Apache-2.0",
   "dependencies": {
     "@eslint/plugin-kit": "^0.2.0",
-    "@humanwhocodes/momoa": "^3.2.1"
+    "@humanwhocodes/momoa": "^3.3.0"
   },
   "devDependencies": {
     "@eslint/core": "^0.6.0",

src/languages/json-language.js

@@ -21,6 +21,10 @@ import { visitorKeys } from "@humanwhocodes/momoa";
 /** @typedef {import("@eslint/core").OkParseResult<DocumentNode>} OkParseResult */
 /** @typedef {import("@eslint/core").ParseResult<DocumentNode>} ParseResult */
 /** @typedef {import("@eslint/core").File} File */
+/**
+ * @typedef {Object} JSONLanguageOptions
+ * @property {boolean} [allowTrailingCommas] Whether to allow trailing commas.
+ */
 
 //-----------------------------------------------------------------------------
 // Exports
@@ -76,26 +80,42 @@ export class JSONLanguage {
 		this.#mode = mode;
 	}
 
-	/* eslint-disable class-methods-use-this, no-unused-vars -- Required to complete interface. */
 	/**
 	 * Validates the language options.
-	 * @param {Object} languageOptions The language options to validate.
+	 * @param {JSONLanguageOptions} languageOptions The language options to validate.
 	 * @returns {void}
 	 * @throws {Error} When the language options are invalid.
 	 */
 	validateLanguageOptions(languageOptions) {
-		// no-op
+		if (languageOptions.allowTrailingCommas !== undefined) {
+			if (typeof languageOptions.allowTrailingCommas !== "boolean") {
+				throw new Error(
+					"allowTrailingCommas must be a boolean if provided.",
+				);
+			}
+
+			// we know that allowTrailingCommas is a boolean here
+
+			// only allowed in JSONC mode
+			if (this.#mode !== "jsonc") {
+				throw new Error(
+					"allowTrailingCommas option is only available in JSONC.",
+				);
+			}
+		}
 	}
-	/* eslint-enable class-methods-use-this, no-unused-vars -- Required to complete interface. */
 
 	/**
 	 * Parses the given file into an AST.
 	 * @param {File} file The virtual file to parse.
+	 * @param {{languageOptions: JSONLanguageOptions}} context The options to use for parsing.
 	 * @returns {ParseResult} The result of parsing.
 	 */
-	parse(file) {
+	parse(file, context) {
 		// Note: BOM already removed
 		const text = /** @type {string} */ (file.body);
+		const allowTrailingCommas =
+			context?.languageOptions?.allowTrailingCommas;
 
 		/*
 		 * Check for parsing errors first. If there's a parsing error, nothing
@@ -108,6 +128,7 @@ export class JSONLanguage {
 				mode: this.#mode,
 				ranges: true,
 				tokens: true,
+				allowTrailingCommas,
 			});
 
 			return {

tests/languages/json-language.test.js

@@ -23,6 +23,55 @@ describe("JSONLanguage", () => {
 		});
 	});
 
+	describe("validateLanguageOptions()", () => {
+		it("should throw an error when allowTrailingCommas is not a boolean", () => {
+			const language = new JSONLanguage({
+				mode: "jsonc",
+				allowTrailingCommas: "true",
+			});
+			assert.throws(() => {
+				language.validateLanguageOptions({
+					allowTrailingCommas: "true",
+				});
+			}, /allowTrailingCommas/u);
+		});
+
+		it("should throw an error when allowTrailingCommas is a boolean in JSON mode", () => {
+			const language = new JSONLanguage({ mode: "json" });
+			assert.throws(() => {
+				language.validateLanguageOptions({ allowTrailingCommas: true });
+			}, /allowTrailingCommas/u);
+		});
+
+		it("should throw an error when allowTrailingCommas is a boolean in JSON5 mode", () => {
+			const language = new JSONLanguage({ mode: "json5" });
+			assert.throws(() => {
+				language.validateLanguageOptions({ allowTrailingCommas: true });
+			}, /allowTrailingCommas/u);
+		});
+
+		it("should not throw an error when allowTrailingCommas is a boolean in JSONC mode", () => {
+			const language = new JSONLanguage({ mode: "jsonc" });
+			assert.doesNotThrow(() => {
+				language.validateLanguageOptions({ allowTrailingCommas: true });
+			});
+		});
+
+		it("should not throw an error when allowTrailingCommas is not provided", () => {
+			const language = new JSONLanguage({ mode: "jsonc" });
+			assert.doesNotThrow(() => {
+				language.validateLanguageOptions({});
+			});
+		});
+
+		it("should not throw an error when allowTrailingCommas is not provided and other keys are present", () => {
+			const language = new JSONLanguage({ mode: "jsonc" });
+			assert.doesNotThrow(() => {
+				language.validateLanguageOptions({ foo: "bar" });
+			});
+		});
+	});
+
 	describe("parse()", () => {
 		it("should not parse jsonc by default", () => {
 			const language = new JSONLanguage({ mode: "json" });
@@ -38,6 +87,49 @@ describe("JSONLanguage", () => {
 			);
 		});
 
+		it("should not parse trailing commas by default in json mode", () => {
+			const language = new JSONLanguage({ mode: "json" });
+			const result = language.parse({
+				body: '{\n"a": 1,\n}',
+				path: "test.json",
+			});
+
+			assert.strictEqual(result.ok, false);
+			assert.strictEqual(
+				result.errors[0].message,
+				"Unexpected token RBrace found.",
+			);
+		});
+
+		it("should not parse trailing commas by default in jsonc mode", () => {
+			const language = new JSONLanguage({ mode: "jsonc" });
+			const result = language.parse({
+				body: '{\n"a": 1,\n}',
+				path: "test.jsonc",
+			});
+
+			assert.strictEqual(result.ok, false);
+			assert.strictEqual(
+				result.errors[0].message,
+				"Unexpected token RBrace found.",
+			);
+		});
+
+		it("should parse trailing commas when enabled in jsonc mode", () => {
+			const language = new JSONLanguage({ mode: "jsonc" });
+			const result = language.parse(
+				{
+					body: '{\n"a": 1,\n}',
+					path: "test.jsonc",
+				},
+				{ languageOptions: { allowTrailingCommas: true } },
+			);
+
+			assert.strictEqual(result.ok, true);
+			assert.strictEqual(result.ast.type, "Document");
+			assert.strictEqual(result.ast.body.type, "Object");
+		});
+
 		it("should parse json by default", () => {
 			const language = new JSONLanguage({ mode: "json" });
 			const result = language.parse({