.eslintrc.js

// The ESLint ecmaVersion argument is inconsistently used. Some rules will ignore it entirely, so if the rule has
// been set, it will still error even if it's not applicable to that version number. Since Google sets these
// rules, we have to turn them off ourselves.
var DISABLED_ES6_OPTIONS = {
  'no-var': 'off',
  'prefer-rest-params': 'off',
  'prefer-spread': 'off',
  // Not supported in ES3
  'comma-dangle': ['error', 'never']
};

var CUSTOM_RULES = {
  'one-var': 'off',
  // We control our own objects and prototypes, so no need for this check
  'guard-for-in': 'off',
  // Google prescribes different indents for different cases. Let's just use 2 spaces everywhere. Note that we have
  // to override ESLint's default of 0 indents for this.
  'indent': ['error', 2, {'SwitchCase': 1}],
  // Less aggressive line length than Google, which is especially useful when we have a lot of callbacks in our code
  'max-len': ['error', {code: 120, tabWidth: 2, ignoreUrls: true}],
  // Go back to default ESLint behaviour here, which is slightly better for catching erroneously unused variables
  'no-unused-vars': ['error', {vars: 'all', args: 'after-used'}],
  'require-jsdoc': 'off',
  'valid-jsdoc': 'off'
};

module.exports = {
  extends: 'google',
  ignorePatterns: ['.gitignore', 'lib/**/*.js'],
  parserOptions: {
    ecmaVersion: 5
  },
  rules: Object.assign(
    {},
    DISABLED_ES6_OPTIONS,
    CUSTOM_RULES
  ),
  overrides: [
    {
      files: ['test/**/*.js'],
      parserOptions: {
        ecmaVersion: 2017
      }
    }
  ]
};