[no-unused-vars] flagged though it is used inside an object

[gforcada]

Tell us about your environment

• ESLint Version: 6.6.0 as well as 7.0.0
• Node Version: 12.16.3
• npm Version: 6.14.4

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

Please show your full configuration:

Configuration

{
  "env": {
    "browser": true,
    "es6": true,
    "jquery": true
  },
  "extends": [
    "airbnb-base",
    "plugin:prettier/recommended"
  ],
  "parser": "babel-eslint",
  "parserOptions": {
    "sourceType": "module"
  },
  "plugins": ["prettier"],
  "rules": {
    "camelcase": "off",
  }
}

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

  let placedAt = 0;
  let placedAt = myFunction({
    previous: placedAt
  });

eslint js/

What did you expect to happen?
eslint finishing without an error.

What actually happened? Please include the actual, raw output from ESLint.
eslint complains with error 'placedAt' is assigned a value but never used no-unused-vars.

While the placedAt variable is used inside the object given to myFunction function

Are you willing to submit a pull request to fix this bug?
I could try, though my time and JS-foo is not so good, but given some guidance I can give it a try.

[platinumazure]

Are you sure you want to redeclare placedAt?

Seems this would make more sense:

  let placedAt = 0;
  placedAt = myFunction({
    previous: placedAt
  });

Also, no-unused-vars is expecting another use of placedAt after the reassignment. With the code you have provided, you could call myFunction without assigning the result, and this would both satisfy the rule and have no observable effect on behavior:

  let placedAt = 0;
   myFunction({
    previous: placedAt
  });
  // Since nothing else is reading placedAt, this is equivalent behavior

If you do intend to reassign to placedAt, simply use it later in the code, and the rule will be satisfied.

[gforcada]

@platinumazure thanks for the quick feedback!

I see that I removed too much to simplify the example, the code looks like this:

  let placedAt = 0;
  placedAt = myFunction({
    previous: placedAt
  });
  placedAt = myFunction({
    previous: placedAt
  });
  placedAt = myFunction({
    previous: placedAt
  });

So the myFunction returns a position, which is needed for the next call.

[platinumazure]

Hi @gforcada,

The logic is the same as before: On the last reassignment, you could just call the function for its side effects, without reassigning the variable.

  let placedAt = 0;
  placedAt = myFunction({
    previous: placedAt
  });
  placedAt = myFunction({
    previous: placedAt
  });
  /* placedAt = */ myFunction({
    previous: placedAt
  });

Or you can add another usage:

  let placedAt = 0;
  placedAt = myFunction({
    previous: placedAt
  });
  placedAt = myFunction({
    previous: placedAt
  });
  placedAt = myFunction({
    previous: placedAt
  });

  doSomethingWith(placedAt);

Basically, the last assignment is what is unused here. Either drop that reassignment, or use the variable again, and the rule will be satisfied.

[gforcada]

Oh I see know 🤦 sorry for the noise and thanks for the (very quick 👍 ) explanations!! 🙇

[gforcada]

Final minor disclaimer that might be worthy: I did report it as eslint was complaining of no-unused-vars on the line where placedAt was being defined, not on the last usage that is indeed superfluous. Maybe that could be updated/improved (the error line where it is reported)? 🤔

[mdjermanovic]

Final minor disclaimer that might be worthy: I did report it as eslint was complaining of no-unused-vars on the line where placedAt was being defined, not on the last usage that is indeed superfluous. Maybe that could be updated/improved (the error line where it is reported)?

There's an enhancement proposal for this: #13181