eslintrc.json

{
  // http://eslint.org/docs/rules/
  // By default, ESLint will look for configuration files in all parent
  // folders up to the root directory. Prevent this by telling ESLint
  // that this is the root of the project.
  "root": true,

  // Which environments your script is designed to run in.
  // Each environment brings with it a certain set of predefined global variables.
  "env": {
    "browser": true,
    "es6": true,
    "jquery": true
  },

  "globals": {
    // Let ESLint know about defined global variables.
    "Drupal": true,
    "drupalSettings": true
  },

  "parserOptions": {
    "ecmaVersion": 9,
    "sourceType": "module"
  },

  // Inherit settings from ESLint Recommended config.
  // Rules above override any rules configured here.
  "extends": "eslint:recommended",

  // Let ESLint know that the Gulpfile and any Gulp
  // tasks run in Node not the browser.
  "overrides": [
    {
      "files": [
        "gulpfile.js",
        "gulp-tasks/**/*.js"
      ],
      "env": {
        "node": true
      }
    }
  ],

  //  0 - turn the rule off
  // 1 - turn the rule on as a warning (doesn't affect exit code)
  // 2 - turn the rule on as an error (exit code is 1 when triggered)
  "rules": {
    // Two space indentation.
    "indent": ["error", 2, { "SwitchCase": 1 }],

    // Prefer single quotes over double.
    "quotes": ["error", "single"],

    // Specify Unix line endings.
    "linebreak-style": ["error", "unix"],

    // Enforce using semicolons.
    "semi": ["error", "always"],

    // Enforce camelcase for variables.
    "camelcase": "error",

    // Prohibit use of == and != in favor of === and !==.
    "eqeqeq": "error",

    // If ESlint considers a file to be a Node module one 'use strict'
    // is needed at the top of the file. If not 'use strict' needs to
    // be added at the top of the function scope.
    "strict": ["off"],

    // Prohibit use of a variable before it is defined.
    "no-undef": "error",

    // Enforce line length to 80 characters.
    "max-len": ["error", {
      "ignoreUrls": true,
      "ignoreStrings": true,
      "ignoreRegExpLiterals": true,
      "ignoreTemplateLiterals": true
    }],

    // Require capitalized names for constructor functions.
    "new-cap": "error",

    // Warn when variables are defined but never used.
    "no-unused-vars": "warn",

    // Require one var declaration for each variable and
    // declare each variable on a newline.
    "one-var": ["error", "never"],

    // Enforce stroustrup style for braces.
    "brace-style": ["error", "stroustrup"],

    // Validates JSDoc comments are syntactically correct
    "valid-jsdoc": "error",

      // Treat var as Block Scoped
    "block-scoped-var": "warn",

    // Require Following Curly Brace Conventions
    "curly": "error",

    // Disallow Use of Alert
    "no-alert": "warn",

    // Disallow eval()
    "no-eval": "error",

    // Disallow the type conversion with shorter notations
    "no-implicit-coercion": "error",

    // Disallow Functions in Loops
    "no-loop-func": "error",

    // Disallow Script URLs
    "no-script-url": "error",

    // Disallow Use of the Comma Operator
    "no-sequences": "error",

    // Disallow unnecessary concatenation of strings
    "no-useless-concat": "error",

    // Disallow Early Use
    "no-use-before-define": "error",

    // Require spacing in object literals.
    // "object-curly-spacing": ["error", "always"],

    // Require file to end with single newline
    "eol-last": "error",

    // Disallow trailing spaces at the end of lines
    "no-trailing-spaces": "error",

    // Disallow Dangling Underscores in Identifiers
    "no-underscore-dangle": "error",

    // Require JSDoc comment
    "require-jsdoc": "error",

    // Require Or Disallow Space Before Blocks
    "space-before-blocks": "error",

    // Disallow Yoda Conditions
    "yoda": "error"
  }
}