Add top-level-interop rule.

Partial fix for eslint#68.

README.md

@@ -156,6 +156,7 @@ export default [
 - `no-empty-keys` - warns when there is a key in an object that is an empty string or contains only whitespace (note: `package-lock.json` uses empty keys intentionally)
 - `no-unsafe-values` - warns on values that are unsafe for interchange, such as numbers outside safe range or lone surrogates.
 - `no-unnormalized-keys` - warns on keys containing [unnormalized characters](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/normalize#description). You can optionally specify the normalization form via `{ form: "form_name" }`, where `form_name` can be any of `"NFC"`, `"NFD"`, `"NFKC"`, or `"NFKD"`.
+- `top-level-interop` - warns when the top-level item in the document is neither an array nor an object. This can be enabled to ensure maximal interoperability with the oldest JSON parsers.
 
 ## Configuration Comments
 

src/index.js

@@ -13,6 +13,7 @@ import noDuplicateKeys from "./rules/no-duplicate-keys.js";
 import noEmptyKeys from "./rules/no-empty-keys.js";
 import noUnsafeValues from "./rules/no-unsafe-values.js";
 import noUnnormalizedKeys from "./rules/no-unnormalized-keys.js";
+import topLevelinterop from "./rules/top-level-interop.js";
 
 //-----------------------------------------------------------------------------
 // Plugin
@@ -33,6 +34,7 @@ const plugin = {
 		"no-empty-keys": noEmptyKeys,
 		"no-unsafe-values": noUnsafeValues,
 		"no-unnormalized-keys": noUnnormalizedKeys,
+		"top-level-interop": topLevelinterop,
 	},
 	configs: {
 		recommended: {
@@ -42,6 +44,7 @@ const plugin = {
 				"json/no-empty-keys": "error",
 				"json/no-unsafe-values": "error",
 				"json/no-unnormalized-keys": "error",
+				"json/top-level-interop": "off",
 			}),
 		},
 	},

src/rules/top-level-interop.js

@@ -0,0 +1,35 @@
+/**
+ * @fileoverview Rule to ensure top-level items are either an array or ojbect.
+ * @author Joe Hildebrand
+ */
+
+export default {
+	meta: {
+		type: /** @type {const} */ ("problem"),
+
+		docs: {
+			description:
+				"Disallow JSON top-level items are not an array or object",
+		},
+
+		messages: {
+			topLevel:
+				'Top level item should be array or object, got "{{type}}".',
+		},
+	},
+
+	create(context) {
+		return {
+			Document(node) {
+				const { type } = node.body;
+				if (type !== "Object" && type !== "Array") {
+					context.report({
+						loc: node.loc,
+						messageId: "topLevel",
+						data: { type },
+					});
+				}
+			},
+		};
+	},
+};

tests/rules/top-level-interop.test.js

@@ -0,0 +1,108 @@
+/**
+ * @fileoverview Tests for top-level-interop rule.
+ * @author Joe Hildebrand
+ */
+
+//------------------------------------------------------------------------------
+// Imports
+//------------------------------------------------------------------------------
+
+import rule from "../../src/rules/top-level-interop.js";
+import json from "../../src/index.js";
+import { RuleTester } from "eslint";
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+	plugins: {
+		json,
+	},
+	language: "json/json",
+});
+
+ruleTester.run("top-level-interop", rule, {
+	valid: [
+		"[]",
+		{
+			code: "[1]",
+			language: "json/json5",
+		},
+		{
+			code: "[1, 2]",
+			language: "json/json5",
+		},
+		"{}",
+		{
+			code: '{"foo": 1}',
+			language: "json/json5",
+		},
+		{
+			code: '{"foo": 1, "foo": 2}',
+			language: "json/json5",
+		},
+	],
+	invalid: [
+		{
+			code: "1",
+			errors: [
+				{
+					messageId: "topLevel",
+					data: {
+						type: "Number",
+					},
+					line: 1,
+					column: 1,
+					endLine: 1,
+					endColumn: 2,
+				},
+			],
+		},
+		{
+			code: "true",
+			errors: [
+				{
+					messageId: "topLevel",
+					data: {
+						type: "Boolean",
+					},
+					line: 1,
+					column: 1,
+					endLine: 1,
+					endColumn: 5,
+				},
+			],
+		},
+		{
+			code: "null",
+			errors: [
+				{
+					messageId: "topLevel",
+					data: {
+						type: "Null",
+					},
+					line: 1,
+					column: 1,
+					endLine: 1,
+					endColumn: 5,
+				},
+			],
+		},
+		{
+			code: '"foo"',
+			errors: [
+				{
+					messageId: "topLevel",
+					data: {
+						type: "String",
+					},
+					line: 1,
+					column: 1,
+					endLine: 1,
+					endColumn: 6,
+				},
+			],
+		},
+	],
+});