[no-unused-vars]: reporter should report the last reference instead of declaration

[anikethsaha]

What rule do you want to change?
no-unused-vars

Does this change cause the rule to produce more or fewer warnings?
same

How will the change be implemented? (New option, new default behavior, etc.)?
default

Please provide some example code that this change will affect:
Currently, the reporter reports the location in a declaration of a variable which is fine but not for all cases.

case 1

const unusedVar = 1; // reporting here is ok

case2

let a = 1 // currently its reporting here,  reporting here is not ok as `a` has references though its being self referred 
a = a + 1;
a = a + 2;
a = a + 3; // it should report here

In this way, it makes easy to track the error if any.
Another advantage of reporting this way is many editors do provide shortcut keys to jump to the declaration of any variable/function but the opposite is not true for some.

ref : #12871, #13172

What does the rule currently do for this code?
reports in the variable declaration

What will the rule do after it's changed?
reports in the last reference of the variable if the variable is being self-referred

Are you willing to submit a pull request to implement this change?
Yes

[nzakas]

Thanks for writing this up. Just to make sure I understand, you’re suggesting that the last reference of an unused variable be highlighted? And if there is only a declaration then the declaration is highlighted?

[anikethsaha]

Yes

[nzakas]

That sounds good to me. It would be nice if we could use a clearer message to, something like "The value of variable foo is never used after this assignment" or something similar.

[anikethsaha]

"The value of variable foo is never used after this assignment"

much better 👍

[Lonniebiz]

This bug confused me (as shown in 13227).

// esLint version 6.7.2
async function fnTest()
{
	return "Promise will resolve to this string.";
}
async function main()
{
	let usedVariable = fnTest(); // esLint thinks usedVariable is unused
	// Omitted: Do other things before awaiting usedVariable
	usedVariable = await usedVariable; // this reassignment is what is unused (not the first line of this function as reported by eslint)
}

eslint reports line 1 of main() as unused, but it is actually line 3 of main() that is unused. eslint should report line 3 of that function-body as "unused reassignment". To me that seems like the most clear thing to call it.

[anikethsaha]

I have submitted the PR #13354 to move this issue forward quickly.