.eslintrc.js

module.exports = {
  root: true,
  env: {
    node: true,
    es6: true,
  },
  overrides: [
    {
      files: ["**/*.ts", "**/*.tsx"],
      parserOptions: {
        parser: "@typescript-eslint/parser",
        ecmaVersion: 2018,
        sourceType: "module",
        ecmaFeatures: {
          impliedStrict: true,
        },
      },
      extends: [
        "airbnb-base", // TURN ON airbnb-base rules.
        "plugin:jest/recommended", // TURN ON Jest rules by using "jest-community/eslint-plugin-jest".
        "plugin:prettier/recommended", // RUN Prettier as ESLint rule by using `prettier/eslint-plugin-prettier` and TURN OFF ESLint rules which conflict with Prettier by using `prettier/eslint-config-prettier`.
        "eslint:recommended", // TURN ON ESLint recommended rules.
        "plugin:@typescript-eslint/eslint-recommended",
        "plugin:@typescript-eslint/recommended", // TURN ON TypeScript rules by using `typescript-eslint/typescript-eslint`.
      ],
      plugins: ["@typescript-eslint"],
      settings: {
        "import/resolver": {
          node: {
            extensions: [".js", ".jsx", ".mjs", ".ts", ".tsx"], // Allow import and resolve for *.ts modules.
          },
        },
      },
      rules: {
        "lines-between-class-members": ["warn", "always", { exceptAfterSingleLine: true }],
        "no-dupe-class-members": "off", // Prevents method overload in TypeScript, and TypeScript already checks duplicates.
        "no-unused-vars": "off", // @typescript-eslint/recommended has same rule
        "no-underscore-dangle": "off",
        "import/prefer-default-export": "off",
        "@typescript-eslint/explicit-function-return-type": ["warn", { allowExpressions: true, allowTypedFunctionExpressions: true }],
        "@typescript-eslint/no-explicit-any": "off",
        "import/extensions": ["error", "ignorePackages", { js: "never", mjs: "never", jsx: "never", ts: "never", tsx: "never" }],
        "@typescript-eslint/no-use-before-define": "off",
        "no-undef-init": "off", // To prevent "Variable is used before being assigned" TS errors.
        "default-param-last": "off",
      },
      overrides: [
        {
          files: ["*.spec.ts", "*.test.ts"],
          env: {
            jest: true,
          },
          rules: {
            "@typescript-eslint/explicit-function-return-type": "off",
          },
        },
      ],
    },
    {
      files: ["*.js"],
      extends: [
        "eslint:recommended", // TURN ON ESLint recommended rules.
        "plugin:jest/recommended", // TURN ON Jest rules by using "jest-community/eslint-plugin-jest".
        "airbnb-base", // TURN ON airbnb-base rules.
        "plugin:prettier/recommended", // RUN Prettier as ESLint rule by using `prettier/eslint-plugin-prettier` and TURN OFF ESLint rules which conflict with Prettier by using `prettier/eslint-config-prettier`.
      ],
      rules: {
        "import/extensions": "off",
      },
    },
  ],
};