Add eslint and prettier (#65)

Source files not reformatted with Prettier in this commit.
Move jest config to external file.

.prettierignore

@@ -0,0 +1,10 @@
+# exclude everything, and opt-in to types we want to format
+**.*
+# add the filetypes we want to format
+!**.js
+!**.mjs
+!**.cjs
+!**.ts
+!**.mts
+!**.cts
+!**.json

.prettierrc.js

@@ -0,0 +1,12 @@
+const config = {
+  // plugins: ['prettier-plugin-jsdoc'],
+  singleQuote: true,
+  overrides: [
+    {
+      files: ['tsconfig*.json'],
+      options: { parser: 'jsonc' },
+    },
+  ],
+};
+
+module.exports = config;

eslint.config.js

@@ -0,0 +1,76 @@
+const globals = require('globals');
+const esLintjs = require('@eslint/js');
+const jest = require('eslint-plugin-jest');
+const tseslint = require('typescript-eslint');
+const prettier = require('eslint-config-prettier');
+//const jsdoc = require('eslint-plugin-jsdoc');
+
+// Using tseslint config helper to customise its setup the tseslint way.
+const tsconfigTsFiles = ['**/*.{ts,mts}'];
+const tsconfigJsFiles = ['**.{js,mjs}'];
+const tseslintConfigs = tseslint.config(
+  {
+    files: tsconfigJsFiles,
+    extends: [...tseslint.configs.recommended],
+    rules: {
+      '@typescript-eslint/no-var-requires': 'off', // (tseslint does not autodetect commonjs context )
+    },
+  },
+  {
+    files: tsconfigTsFiles,
+    extends: [...tseslint.configs.recommended],
+  },
+);
+
+module.exports = [
+  esLintjs.configs.recommended,
+  // jsdoc.configs['flat/recommended'],
+  jest.configs['flat/recommended'],
+  ...tseslintConfigs,
+  prettier, // Do Prettier last so it can override previous configs.
+
+  // Customise rules.
+  {
+    files: ['**/*.{js,mjs,cjs}', '**/*.{ts,mts,cts}'],
+    rules: {
+      'no-else-return': ['error', { allowElseIf: false }],
+
+      // 'jsdoc/tag-lines': 'off',
+      // 'jsdoc/require-jsdoc': 'off',
+      // 'jsdoc/require-param-description': 'off',
+      // 'jsdoc/require-returns-description': 'off',
+    },
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+  },
+  {
+    files: ['tests/*.{js,mjs,cjs,ts,mts,cts}'],
+    rules: {
+      'no-unused-vars': 'off',
+      '@typescript-eslint/no-unused-vars': 'off',
+    },
+  },
+  {
+    files: ['tests/*.test-d.ts'],
+    rules: {
+      'no-constant-condition': 'off', // using `if ('explanation')` for test blocks
+    },
+  },
+  {
+    files: [...tsconfigTsFiles, ...tsconfigJsFiles],
+    rules: {
+      '@typescript-eslint/ban-ts-comment': [
+        'error',
+        {
+          'ts-expect-error': 'allow-with-description',
+          'ts-ignore': 'allow-with-description',
+          'ts-nocheck': true,
+          'ts-check': true,
+        },
+      ],
+    },
+  },
+];

index.d.ts

@@ -452,6 +452,8 @@ export class CommanderError extends Error {
 
   export type OptionValues = Record<string, unknown>;
 
+  // eslint unimpressed with `OptionValues = {}`, but not sure what to use instead.
+  // eslint-disable-next-line @typescript-eslint/ban-types
   export class Command<Args extends any[] = [], Opts extends OptionValues = {}> {
     args: string[];
     processedArgs: Args;

index.js

@@ -1,7 +1,5 @@
 const commander = require('commander');
 
-// @ts-check
-
 exports = module.exports = {};
 
 // Return a different global program than commander,

jest.config.js

@@ -0,0 +1,10 @@
+const config = {
+  testEnvironment: 'node',
+  collectCoverage: true,
+  transform: {
+    '^.+\\.tsx?$': ['ts-jest'],
+  },
+  testPathIgnorePatterns: ['/node_modules/'],
+};
+
+module.exports = config;