README.md

ESLint JSON Language Plugin

Overview

This package contains a plugin that allows you to natively lint JSON and JSONC files using ESLint.

Important: This plugin requires ESLint v9.6.0 or higher and you must be using the new configuration system.

Installation

For Node.js and compatible runtimes:

npm install @eslint/json -D
# or
yarn add @eslint/json -D
# or
pnpm install @eslint/json -D
# or
bun install @eslint/json -D

For Deno:

deno add @eslint/json

Usage

This package exports these languages:

• "json/json" is for regular JSON files
• "json/jsonc" is for JSON files that support comments (JSONC) such as those used for Visual Studio Code configuration files
• "json/json5" is for JSON5 files

Depending on which types of JSON files you'd like to lint, you can set up your eslint.config.js file to include just the files you'd like. Here's an example that lints JSON, JSONC, and JSON5 files:

import json from "@eslint/json";

export default [
	{
		plugins: {
			json,
		},
	},

	// lint JSON files
	{
		files: ["**/*.json"],
		language: "json/json",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},

	// lint JSONC files
	{
		files: ["**/*.jsonc", ".vscode/*.json"],
		language: "json/jsonc",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},

	// lint JSON5 files
	{
		files: ["**/*.json5"],
		language: "json/json5",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},
];

In CommonJS format:

const json = require("@eslint/json").default;

module.exports = [
	{
		plugins: {
			json,
		},
	},

	// lint JSON files
	{
		files: ["**/*.json"],
		language: "json/json",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},

	// lint JSONC files
	{
		files: ["**/*.jsonc", ".vscode/*.json"],
		language: "json/jsonc",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},

	// lint JSON5 files
	{
		files: ["**/*.json5"],
		language: "json/json5",
		rules: {
			"json/no-duplicate-keys": "error",
		},
	},
];

Recommended Configuration

To use the recommended configuration for this plugin, specify your matching files and then use the json.configs.recommended object, like this:

import json from "@eslint/json";

export default [
	// lint JSON files
	{
		files: ["**/*.json"],
		ignores: ["package-lock.json"],
		language: "json/json",
		...json.configs.recommended,
	},

	// lint JSONC files
	{
		files: ["**/*.jsonc"],
		language: "json/jsonc",
		...json.configs.recommended,
	},

	// lint JSON5 files
	{
		files: ["**/*.json5"],
		language: "json/json5",
		...json.configs.recommended,
	},
];

Note: You generally want to ignore package-lock.json because it is auto-generated and you typically will not want to manually make changes to it.

Rules

• no-duplicate-keys - warns when there are two keys in an object with the same text.
• 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 strings with unmatched surrogates, numbers that evaluate to Infinity, numbers that evaluate to zero unintentionally, numbers that look like integers but are too large, and subnormal numbers.
• no-unnormalized-keys - warns on keys containing unnormalized characters. 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

In JSONC and JSON5 files, you can also use rule configurations comments and disable directives.

/* eslint json/no-empty-keys: "error" */

{
	"foo": {
		"": 1, // eslint-disable-line json/no-empty-keys -- We want an empty key here
	},
	"bar": {
		// eslint-disable-next-line json/no-empty-keys -- We want an empty key here too
		"": 2,
	},
	/* eslint-disable json/no-empty-keys -- Empty keys are allowed in the following code as well */
	"baz": [
		{
			"": 3,
		},
		{
			"": 4,
		},
	],
	/* eslint-enable json/no-empty-keys -- re-enable now */
}

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:

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?

This plugin implements JSON parsing for ESLint using the language plugins API, which is the official way of supporting non-JavaScript languages in ESLint. This differs from the other plugins:

• eslint-plugin-json uses a processor to parse the JSON, meaning it doesn't create an AST and you can't write custom rules for it.
• eslint-plugin-jsonc uses a parser that still goes through the JavaScript linting functionality and requires several rules to disallow valid JavaScript syntax that is invalid in JSON.

As such, this plugin is more robust and faster than the others. You can write your own custom rules when using the languages in this plugin, too.

What about missing rules that are available in eslint-plugin-json and eslint-plugin-jsonc?

Most of the rules in eslint-plugin-json are actually syntax errors that are caught automatically by the parser used in this plugin.

Similarly, many of the rules in eslint-plugin-jsonc specifically disallow valid JavaScript syntax that is invalid in the context of JSON. These are also automatically caught by the parser in this plugin.

Any other rules that catch potential problems in JSON are welcome to be implemented. You can open an issue to propose a new rule.

License

Apache 2.0

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