.eslintrc

{
  "env": {
    "es6": true,
    "node": true
  },
  "parserOptions": {
    "ecmaVersion": 6
  }
}

index1.js

// no error detected on line 9/10
function y(callback) {
  setTimeout(() => callback(), 0);
}

function x(a, callback) {
  console.log('STEP 1', a);
  y(() => {
    console.log('STEP 2', a);
    const {a} = () => ({a: 5});
    callback(null, a);
  });
}

x(4, (err, result) => {
  console.log('ERROR', err);
  console.log('RESULT', result);
});

index2.js

// properly reports Parsing error: Identifier 'a' has already been declared on line 3
function x(a, callback) {
  console.log('STEP 1', a);
  const {a} = () => ({a: 5});
  callback(null, a);
}

x(4, (err, result) => {
  console.log('ERROR', err);
  console.log('RESULT', result);
});