.eslintrc.js

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

const eslintRules = {
    'arrow-body-style': 'warn', // WARN b/c blocks style allows for readability and ensure scope
    'arrow-spacing': 'error',
    'eol-last': 'error',
    'eqeqeq': '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': ['error', { allow: ['error', '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 the typescript linter rule is used
    '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 eslintRulesOnlyTypescript = {
    'no-undef': 'off' // Typescript handles undefined variables better than eslint
}

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-function-return-type': 'warn', // Warn b/c return types are not supported in Svelte markdown
    '@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': 'warn', // Warn b/c we have existing code in migration that I don't want to touch to pass new linting rules
    '@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-unsafe-argument': 'off', // OFF b/c ESlint resolves types of the absolute imports as any
    '@typescript-eslint/unused-export-let': 'off', // OFF b/c used in Svelte components for UI logic
    '@typescript-eslint/no-unused-vars': 'error',
    '@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',
    '@typescript-eslint/no-namespace': 'off', // OFF b/c used in Svelte components for exporting types inside of a namespace
}

const linterRules = {
    ...eslintRules,
    ...eslintRulesOnlyTypescript,
    ...typescriptEslintRules,
}

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: './',
            },
            plugins: ['@typescript-eslint', 'svelte3'],
            rules: linterRules,
            settings: svelteSettings,
        },
        {
            files: '**/*.svelte',
            processor: 'svelte3/svelte3',
            settings: svelteSettings,
            rules: {
                ...linterRules,
                ...svelteRules,
            }
        },
    ],
    parser: '@babel/eslint-parser',
    parserOptions: {
        ...parserOptions,
        requireConfigFile: false,
    },
    rules: {
        ...eslintRules,
    },
}