.eslintrc.js

const parserOptions = {
  ecmaVersion: 6,
  sourceType: 'module',
}

const eslintRules = {
  'arrow-body-style': 'error',
  'arrow-spacing': 'error',
  'eol-last': 'error',
  'func-call-spacing': 'error',
  indent: 'off', // OFF b/c causes problems between Prettier and ESLint
  'linebreak-style': 'off', // OFF b/c Windows (Git) puts CRLF line endings
  'missing-declaration': 'off', // OFF b/c throws errors on imports / require statements
  'multiline-ternary': 'off', // OFF b/c causes problems between Prettier and ESLint
  'no-alert': 'error',
  'no-async-promise-executor': 'error',
  'no-case-declarations': 'error',
  'no-console': 'warn',
  'no-control-regex': 'error',
  'no-dupe-keys': 'error',
  'no-empty': 'error',
  'no-extra-boolean-cast': 'error',
  'no-extra-parens': 'off', // OFF b/c reactive dependencies in .svelte files (mostly for errors)
  'no-extra-semi': 'error',
  'no-fallthrough': 'error',
  'no-import-assign': 'error',
  'no-irregular-whitespace': 'error',
  'no-prototype-builtins': 'error',
  'no-return-await': 'error',
  'no-trailing-spaces': 'error',
  'no-useless-escape': 'error',
  'no-undef': 'error',
  'no-underscore-dangle': 'off', // OFF b/c this syntax is used for defining local callback methods
  'no-unreachable': 'error',
  'no-unused-export-let': 'off', // OFF b/c troublesome with some .js files in packages/shared
  'no-unused-vars': 'off', // OFF b/c there are simply too many and they're harmless
  'no-var': 'error',
  'prefer-arrow-callback': 'warn',
  'prefer-const': 'warn',
  'prefer-destructuring': 'off', // OFF b/c it's not really correct
  quotes: ['error', 'single'],
  semi: 'off', // OFF b/c we aren't using semicolons
  'space-before-function-paren': 'off', // OFF b/c we aren't using spaces before function parameters / signatures
  'spaced-comment': 'error',
}

const typescriptEslintRules = {
  '@typescript-eslint/array-type': 'error',
  '@typescript-eslint/await-thenable': 'error',
  '@typescript-eslint/ban-types': 'error',
  '@typescript-eslint/ban-ts-comment': 'warn',
  '@typescript-eslint/explicit-module-boundary-types': 'error',
  '@typescript-eslint/no-array-constructor': 'error',
  '@typescript-eslint/no-empty-function': 'off', // OFF b/c we use empty functions a lot (esp. for initialization)
  '@typescript-eslint/no-explicit-any': 'error',
  '@typescript-eslint/no-extra-non-null-assertion': 'error',
  '@typescript-eslint/no-extra-semi': 'error',
  '@typescript-eslint/no-floating-promises': 'error',
  '@typescript-eslint/no-implied-eval': 'error',
  '@typescript-eslint/no-inferrable-types': 'off', // OFF b/c this errors on some useful code annotations for function signatures
  '@typescript-eslint/no-misused-new': 'error',
  '@typescript-eslint/no-misused-promises': 'error',
  '@typescript-eslint/no-non-null-asserted-optional-chain': 'error',
  '@typescript-eslint/no-non-null-assertion': 'error',
  '@typescript-eslint/no-this-alias': 'error',
  '@typescript-eslint/no-unnecessary-type-assertion': 'error',
  '@typescript-eslint/no-unsafe-assignment': 'off', // OFF b/c used in Svelte components for UI logic
  '@typescript-eslint/no-unsafe-call': 'off', // OFF b/c used in Svelte components for UI logic
  '@typescript-eslint/no-unsafe-member-access': 'off', // OFF b/c there are simply too many linting errors
  '@typescript-eslint/no-unsafe-return': 'off', // OFF b/c used in Svelte components for UI logic
  '@typescript-eslint/no-unused-vars': 'off', // OFF b/c there are simply too many and they're harmless
  '@typescript-eslint/no-var-requires': 'error',
  '@typescript-eslint/prefer-regexp-exec': 'error',
  '@typescript-eslint/restrict-plus-operands': 'off', // OFF b/c not entirely accurate despite proper typings
  '@typescript-eslint/restrict-template-expressions': 'off', // OFF b/c using any is useful in template expressions
  '@typescript-eslint/require-await': 'error',
  '@typescript-eslint/unbound-method': 'error',
}

const svelteRules = {
  '@typescript-eslint/no-explicit-any': 'off', // OFF b/c used for callback methods in Svelte components
}

const svelteSettings = {
  'svelte3/typescript': () => require('typescript'),
  'svelte3/ignore-styles': () => true,
  'svelte3/ignore-warnings': () => false,
}

module.exports = {
  env: {
      browser: true,
      es6: true,
      node: true,
  },
  extends: ['eslint:recommended'],
  overrides: [
      {
          files: ['**/*.ts', '**/*.svelte'],
          extends: [
              'plugin:@typescript-eslint/eslint-recommended',
              'plugin:@typescript-eslint/recommended',
              'plugin:@typescript-eslint/recommended-requiring-type-checking',
          ],
          parser: '@typescript-eslint/parser',
          parserOptions: {
              ...parserOptions,
              extraFileExtensions: ['.svelte'],
              project: './tsconfig.lint.json',
              tsconfigRootDir: __dirname,
          },
          plugins: ['@typescript-eslint', 'svelte3'],
          rules: {
              ...eslintRules,
              ...typescriptEslintRules,
          },
          settings: svelteSettings,
      },
      {
          files: '**/*.svelte',
          processor: 'svelte3/svelte3',
          settings: svelteSettings,
          rules: {
              ...eslintRules,
              ...typescriptEslintRules,
              ...svelteRules,
          }
      },
  ],
  parser: '@babel/eslint-parser',
  parserOptions: {
      ...parserOptions,
      requireConfigFile: false,
  },
  rules: {
      ...eslintRules,
  },
}