feat: Add top-level-interop rule.

A new rule that checks that the top-level item in
a JSON file is either an array or an object, as
suggested by RFC8259.  Not turned on in the recommended
config because the software that had this issue
is almost entirely obsolete.

Refs 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,
+				},
+			],
+		},
+	],
+});