feat(valid-main): add new rule for validating main (#1382)

<!-- 👋 Hi, thanks for sending a PR to eslint-plugin-package-json! 🗂 Please fill out all fields below and make sure each item is true and [x] checked. Otherwise we may not be able to review your PR. --> ## PR Checklist - [x] Addresses an existing open issue: fixes #831 - [x] That issue was marked as [`status: accepting prs`](https://github.com/JoshuaKGoldberg/eslint-plugin-package-json/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22) - [x] Steps in [CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/eslint-plugin-package-json/blob/main/.github/CONTRIBUTING.md) were taken ## Overview This change adds a new `valid-main` rule that uses `validateMain` from package-json-validator to identify errors. It's included in the `recommended` config.

Author: michaelfaith

README.md

@@ -226,6 +226,7 @@ The default settings don't conflict, and Prettier plugins can quickly fix up ord
 | [valid-keywords](docs/rules/valid-keywords.md) | Enforce that the `keywords` property is valid. | ✔️ ✅ | | | |
 | [valid-license](docs/rules/valid-license.md) | Enforce that the `license` property is valid. | ✔️ ✅ | | | |
 | [valid-local-dependency](docs/rules/valid-local-dependency.md) | Checks existence of local dependencies in the package.json | | | | ❌ |
+| [valid-main](docs/rules/valid-main.md) | Enforce that the `main` property is valid. | ✔️ ✅ | | | |
 | [valid-name](docs/rules/valid-name.md) | Enforce that package names are valid npm package names | ✔️ ✅ | | | |
 | [valid-optionalDependencies](docs/rules/valid-optionalDependencies.md) | Enforce that the `optionalDependencies` property is valid. | ✔️ ✅ | | | |
 | [valid-package-definition](docs/rules/valid-package-definition.md) | Enforce that package.json has all properties required by the npm spec | ✔️ ✅ | | | |

docs/rules/valid-main.md

@@ -0,0 +1,23 @@
+# valid-main
+
+💼 This rule is enabled in the following configs: ✔️ `legacy-recommended`, ✅ `recommended`.
+
+<!-- end auto-generated rule header -->
+
+This rule checks that the `main` property is a non-empty string.
+
+Example of **incorrect** code for this rule:
+
+```json
+{
+"main": ["index.js", "secondary.js"]
+}
+```
+
+Example of **correct** code for this rule:
+
+```json
+{
+"main": "index.js"
+}
+```

src/rules/valid-properties.ts

@@ -12,6 +12,7 @@ import {
 validateHomepage,
 validateKeywords,
 validateLicense,
+validateMain,
 validateScripts,
 validateType,
 } from "package-json-validator";
@@ -49,6 +50,7 @@ const properties = [
 ["homepage", validateHomepage],
 ["keywords", validateKeywords],
 ["license", validateLicense],
+["main", validateMain],
 ["optionalDependencies", validateDependencies],
 ["peerDependencies", validateDependencies],
 ["scripts", validateScripts],

src/tests/rules/valid-main.test.ts

@@ -0,0 +1,68 @@
+import { rules } from "../../rules/valid-properties.ts";
+import { ruleTester } from "./ruleTester.ts";
+
+ruleTester.run("valid-main", rules["valid-main"], {
+invalid: [
+{
+code: `{
+"main": null
+}
+`,
+errors: [
+{
+data: {
+error: "the value is `null`, but should be a `string`",
+},
+line: 2,
+messageId: "validationError",
+},
+],
+},
+{
+code: `{
+"main": 123
+}
+`,
+errors: [
+{
+data: {
+error: "the type should be a `string`, not `number`",
+},
+line: 2,
+messageId: "validationError",
+},
+],
+},
+{
+code: `{
+"main": []
+}
+`,
+errors: [
+{
+data: {
+error: "the type should be a `string`, not `Array`",
+},
+line: 2,
+messageId: "validationError",
+},
+],
+},
+{
+code: `{
+"main": ""
+}
+`,
+errors: [
+{
+data: {
+error: "the value is empty, but should be the path to the package's main module",
+},
+line: 2,
+messageId: "validationError",
+},
+],
+},
+],
+valid: ["{}", `{ "main": "./index.js" }`, `{ "main": "index.js" }`],
+});