Stop using mjs file extension

Author: JoshMock

specification/eslint.config.mjs renamed to specification/eslint.config.js

@@ -1,21 +1,45 @@
 # es-spec-validator
 
 An [ESLint](https://eslint.org/) plugin that uses [typescript-eslint](https://typescript-eslint.io/) to validate the Elasticsearch specification against a set of custom rules.
-It is configured [in the specification directory](../specification/eslint.config.mjs).
+It is configured [in the specification directory](../specification/eslint.config.js).
 
 ## Rules
 
 | Name | Description |
 | - | - |
 | `single-key-dictionary-key-is-string` | `SingleKeyDictionary` keys must be strings |
 
-## Writing rules
+## Usage
 
-Each rule should be written in a separate JavaScript file (e.g. `single-key-dictionary-key-is-string.mjs`) following the format of a custom rule [as defined by the typescript-eslint docs](https://typescript-eslint.io/developers/custom-rules).
-Within a rule's `create` function, return an object whose keys are the names of [AST node types](https://typescript-eslint.io/packages/typescript-estree/ast-spec).
+Prerequisites: [Node.js](https://nodejs.org) and [`npm`](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm) must be installed locally.
 
-If a rule needs to report a problem, it should call `context.report()` with the appropriate arguments.
+1. Enter the `specification/` directory: `cd specification/`
+1. Install dependencies (first time only): `npm install`
+1. Run the linter on the specification project: `npm run lint`
+
+Or, from the repository's root directory, run `make setup && make validate`.
+This will run the linter, along with several other validations.
+
+If the linter finds problems in the spec code, it will print them out.
+If all is well, the output will be minimal, and the process will exit cleanly.
+
+ESLint also maintains [integrations](https://eslint.org/docs/latest/use/integrations#editors) for most common code editors to provide instant inline feedback as you modify files.
+
+## Writing new linter rules
+
+Each rule should be written in a separate JavaScript file (e.g. `single-key-dictionary-key-is-string.js`) following the format of a custom rule [as defined by the typescript-eslint docs](https://typescript-eslint.io/developers/custom-rules).
+Within a rule's `create` function, return an object whose keys are the names of [AST node types](https://typescript-eslint.io/packages/typescript-estree/ast-spec) and whose values are functions that validate that node type.
+To report a problem in a validation function, it should call `context.report()` with the appropriate arguments.
 
 To get a familiar the different node types possible within a TypeScript AST, paste some code into [an AST viewer](https://ts-ast-viewer.com/) and explore the resulting visual tree.
 
-To add your rule to the spec validator ESLint plugin, import it into [the plugin file](./eslint-plugin-es-spec.mjs) and add it to the `rules` object with an appropriate key.
+To add your rule to the spec validator ESLint plugin, import it into [the plugin file](./eslint-plugin-es-spec.js) and add it to the `rules` object with an appropriate key.
+
+### ESM format
+
+The validator project is written in standard JavaScript rather than TypeScript, and uses the new [ESM](https://nodejs.org/api/esm.html) module format for managing module imports and exports.
+Files should use the normal `.js` file extension, and the `import` format is almost identical to TypeScript's, except that ESM requires you to include the file extension in the import statement. Example:
+
+```js
+import myRule from './rules/my-rule.js' // not ./rules/my-rule
+```

specification/package-lock.json

@@ -0,0 +1,7 @@
+import singleKeyDict from './rules/single-key-dictionary-key-is-string.js'
+
+export default {
+  rules: {
+    'single-key-dictionary-key-is-string': singleKeyDict
+  }
+}

validator/README.md

@@ -2,7 +2,8 @@
   "name": "eslint-plugin-es-spec",
   "version": "0.1.0",
   "description": "Elasticsearch specification validation",
-  "main": "eslint-plugin-es-spec.mjs",
+  "main": "eslint-plugin-es-spec.js",
+  "type": "module",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1"
   },