prefer-const and conditional return

Tell us about your environment

• ESLint Version: 3.19
• Node Version: 8.5
• npm Version: 5.5.1 What parser (default, Babel-ESLint, etc.) are you using? default Please show your full configuration:

Configuration

{
    "parserOptions": {
        "ecmaVersion": 6,
        "sourceType": "script",
        "ecmaFeatures": {}
    },
    "rules": {
        "constructor-super": 2,
        "no-case-declarations": 2,
        "no-class-assign": 2,
        "no-compare-neg-zero": 2,
        "no-cond-assign": 2,
        "no-console": 2,
        "no-const-assign": 2,
        "no-constant-condition": 2,
        "no-control-regex": 2,
        "no-debugger": 2,
        "no-delete-var": 2,
        "no-dupe-args": 2,
        "no-dupe-class-members": 2,
        "no-dupe-keys": 2,
        "no-duplicate-case": 2,
        "no-empty-character-class": 2,
        "no-empty-pattern": 2,
        "no-empty": 2,
        "no-ex-assign": 2,
        "no-extra-boolean-cast": 2,
        "no-extra-semi": 2,
        "no-fallthrough": 2,
        "no-func-assign": 2,
        "no-global-assign": 2,
        "no-inner-declarations": 2,
        "no-invalid-regexp": 2,
        "no-irregular-whitespace": 2,
        "no-mixed-spaces-and-tabs": 2,
        "no-new-symbol": 2,
        "no-obj-calls": 2,
        "no-octal": 2,
        "no-redeclare": 2,
        "no-regex-spaces": 2,
        "no-self-assign": 2,
        "no-sparse-arrays": 2,
        "no-this-before-super": 2,
        "no-undef": 2,
        "no-unexpected-multiline": 2,
        "no-unreachable": 2,
        "no-unsafe-finally": 2,
        "no-unsafe-negation": 2,
        "no-unused-labels": 2,
        "no-unused-vars": 2,
        "no-useless-escape": 2,
        "require-yield": 2,
        "use-isnan": 2,
        "valid-typeof": 2,
        "prefer-const": 2
    },
    "env": {}
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

function fn() {
  let foo;
  if (Math.random() > .5) {
    foo = 'hop';
    return foo;
  }
  foo = 'baz';
  return foo;
}
fn();

eslint filename

What did you expect to happen? I expected ESlint to pickup on the fact that either we enter the if, in which case the return makes it so the foo var is never reassigned, or we don’t, in which case the foo bar is also never reassigned.

What actually happened? Please include the actual, raw output from ESLint. ESlint doesn’t report an error

[not-an-aardvark]

I don’t think this would be in scope for the prefer-const rule. The goal of the rule is to report let declarations that can be replaced with const, or moved to a different location and then replaced with const. However, the rule is not intended to tell the user when they could create a new variable to accomplish the same thing.

[not-an-aardvark]

Hi, thanks for the report.

I’m confused about the behavior that you were expecting. There are two locations where foo could be assigned, even though only one of them will actually be selected at runtime. This means that it wouldn’t be possible to declare foo with const, so I wouldn’t expect prefer-const to report an error for this case.

Simpler example:

let foo;
if (x) {
  foo = 1;
} else {
  foo = 2;
}
console.log(foo);

Similarly, in this example foo will only get assigned once at runtime, but it doesn’t seem like it would be possible to declare foo with const (assuming we keep the general structure of the code rather than reducing it to a ternary expression).