no-unused-vars false positive for use on right side of assignment

Tell us about your environment

• ESLint Version: http://eslint.org/demo
• Node Version: n/a
• npm Version: n/a

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

Default

Please show your full configuration:

None, inline

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

/* eslint no-unused-vars: "error" */
// 'foo' is defined but never used. (no-unused-vars)
var foo;
foo = 42;
foo = foo();
foo = foo.bar();
foo = bar(foo);

// No error
var bar;
bar = 42;
bar();

What did you expect to happen?

No error. foo is used multiple times on the right side of the assignment expressions.

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

3:5 - ‘foo’ is defined but never used. (no-unused-vars)

It looks like references are being excluded on both sides of assignment expressions instead of just the left side.

Originally seen on http://stackoverflow.com/questions/39787723/eslint-no-unused-vars-stream-conditional-pipe-use-case

[btmills]

The left side of the final assignment expression is unnecessary, but the variable is “used”, at least according to my interpretation of the rule. You can look at the linked StackOverflow question for a less trivial example. In an ideal world (and I believe this is what you were getting at when you suggested warning on line 7 instead of the declaration), we’d be able to determine statically that the result of the final assignment expression is unused and suggest using the right side by itself. Even supposing that were possible (it may be, and that be really cool), I don’t think that warning should come from no-unused-vars. Put another way, my interpretation of no-unused-vars is that when it declares a variable as unused, I ought to be able to remove that variable from my program without changing the program’s behavior. Does that fit?

Also, welcome to the team! 🎉

[nzakas]

@not-an-aardvark I’m open to changing the message that’s reported in order to avoid confusion in those cases. Maybe something like “foo is assigned a value but is never used” instead?

@mysticatea no need to apologize. It’s just that you didn’t leave a comment explaining why you removed “accepted,” so I wasn’t sure if you disagreed that this was a bug, or something else.