Demonstration of a subtle ESLint parser conflict with "prefer-const"

.eslintconfig.json

{
  "parserOptions": {
    "ecmaVersion": 6,
    "sourceType": "module",
  },
  "rules": {
    "prefer-const": 2
  }
}

eslint.log

eslint -c .eslint.test.config test/prefer-const.js 

prefer-const.js
  7:5  error  'third' is never modified, use 'const' instead  prefer-const

✖ 1 problem (1 error, 0 warnings)

output.log

# Simple as possible repo
$ node test/prefer-const.js 
third = 0.10418695327825844
third = 0.8143592826090753

#
# Running the slightly more complicated mocha example
# demonstrates the common-place nature of the pattern.
#
$ mocha test/prefer-const.js 

  test suite
third = 0.9538288279436529
    ✓ some test
third = 0.2518426834139973
    ✓ some other test


  2 passing (7ms)

prefer-const-fail.test.js

'use strict';

const assert = require('assert');

describe('test suite', function () {
  let first;
  let second;
  let third;

  function setupTest() {
    first = Math.random();
    second = Math.random();
    third = Math.random();
    console.log('third = %s', third);
  }

  it('some test', function () {
    setupTest();
    assert.equal(typeof first, 'number');
    assert.equal(typeof second, 'number');
  });

  it('some other test', function () {
    setupTest();
    assert.equal(typeof first, 'number');
    assert.equal(typeof second, 'number');
  });
});

prefer-const.js

'use strict';

const assert = require('assert');

let first;
let second;
let third;

function setupTest() {
  first = Math.random();
  second = Math.random();
  third = Math.random();
  console.log('third = %s', third);
}

setupTest();
assert.equal(typeof first, 'number');
assert.equal(typeof second, 'number');

setupTest();
assert.equal(typeof first, 'number');
assert.equal(typeof second, 'number');