prefer-const false positive

[SimenB]

What version of ESLint are you using?
2.4.0

What parser (default, Babel-ESLint, etc.) are you using?
default

Please show your full configuration:

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

What did you do? Please include the actual source code causing the issue.

import React from 'react';
import { browser } from '../../util/browserUtil';

let DevTools;

if (!browser.ie8()) DevTools = require('../devTools/devTools');

export default ({ children }) => (
  <div>
    <div>
      {children}
    </div>
    {DevTools && <DevTools />}
  </div>
);

What did you expect to happen?
No warning, as DevTools is used outside of the if-statement

What actually happened? Please include the actual, raw output from ESLint.

/home/testyo/repos/meeeeeeep/file.js
  6:5  error  'DevTools' is never reassigned, use 'const' instead  prefer-const

I'm aware of many potentially related issues (#5251, #5283, and #5284), but I don't think this one is covered

[btmills]

This is interesting. Tried these in the demo:

Valid:

/* eslint prefer-const: 2 */
let foo;
if (bar) {
    foo = 42;
}
baz(foo);

Invalid (valid example with braces around if body removed):

/* eslint prefer-const: 2 */
let foo;
if (bar) foo = 42;
baz(foo);

2:5 - 'foo' is never reassigned, use 'const' instead. (prefer-const)

Invalid (valid example with function call removed):

/* eslint prefer-const: 2 */
let foo;
if (bar) {
    foo = 42;
}

2:5 - 'foo' is never reassigned, use 'const' instead. (prefer-const)

Why is it that the combination of curly braces around the if with a function call is considered valid, but if I remove either it errors?

@SimenB if you add curly braces around the if body, does it stop warning for you too?

[SimenB]

@btmills Ah, good catch! Adding braces makes the warning go away

[btmills]

Cool. So at least there's a workaround that can be used while we figure out what's going on here.

I don't see any tests that cover this case. @mysticatea you looked at scoping for prefer-const recently - any ideas as to what's going on here?

[mysticatea]

I has forgotten this case:

/* eslint prefer-const: 2 */
let foo;
if (bar) foo = 42;
baz(foo);