old_contents: let optional = 1;
  let required = 2;
  
  module.exports = {
    parserOptions: {
      sourceType: 'module',
    },
    env: {
      node: true,
      es6: true,
      mocha: true,
    },
    globals: {
      expect: true,
    },
    rules: {
      semi: [required, 'never'],
      'comma-dangle': [required, 'always-multiline'],
      'comma-style': [required, 'last'],
      indent: [required, 2],
      curly: [required, 'all'],
      'brace-style': [required, '1tbs'],
      quotes: [required, 'single'],
      'object-curly-spacing': [required, 'never'],
      'array-bracket-spacing': [required, 'never'],
      // 'space-infix-ops': [required, {int32Hint: false}],
      'no-var': required,
      strict: required,
      'no-undef': required,
      'no-unused-vars': [required, {vars: 'all', args: 'after-used'}],
      'block-scoped-var': required,
      camelcase: required,
      'eol-last': required,
      eqeqeq: [required, 'smart'],
      'max-depth': [optional, 3],
      'max-len': [optional, 120],
      'max-statements': [optional, 15],
      // 'no-extend-native': required,
      'no-mixed-spaces-and-tabs': required,
      'no-trailing-spaces': required,
      'no-multiple-empty-lines': [required, {max: 1, maxBOF: 0}],
      'block-spacing': [required, 'never'],
      'newline-per-chained-call': required,
      'keyword-spacing': required,
      'key-spacing': [required, {afterColon: true}],
      'space-unary-ops': required,
    }
  };
  
new_contents: 'use strict'
  
  let optional = 1;
  let required = 2;
  
  module.exports = {
    parserOptions: {
      sourceType: 'module',
    },
    env: {
      node: true,
      es6: true,
      mocha: true,
    },
    globals: {
      expect: true,
    },
    rules: {
      semi: [required, 'never'],
      'comma-dangle': [required, 'always-multiline'],
      'comma-style': [required, 'last'],
      indent: [required, 2],
      curly: [required, 'all'],
      'brace-style': [required, '1tbs'],
      quotes: [required, 'single'],
      'object-curly-spacing': [required, 'never'],
      'array-bracket-spacing': [required, 'never'],
      // 'space-infix-ops': [required, {int32Hint: false}],
      'no-var': required,
      strict: required,
      'no-undef': required,
      'no-unused-vars': [required, {vars: 'all', args: 'after-used'}],
      'block-scoped-var': required,
      camelcase: required,
      'eol-last': required,
      eqeqeq: [required, 'smart'],
      'max-depth': [optional, 3],
      'max-len': [optional, 120],
      'max-statements': [optional, 15],
      // 'no-extend-native': required,
      'no-mixed-spaces-and-tabs': required,
      'no-trailing-spaces': required,
      'no-multiple-empty-lines': [required, {max: 1, maxBOF: 0}],
      'block-spacing': [required, 'never'],
      'newline-per-chained-call': required,
      'keyword-spacing': required,
      'key-spacing': [required, {afterColon: true}],
      'space-unary-ops': required,
    }
  };
  
subject: use strict inside .eslintrc.js