no-unused-vars does not report error when comma operator is used

With no-unused-vars enabled eslint does not mark a variable as unused if assignment is combined with the comma operator.

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

Both produce no warning:

let x = 0;
x++, x = 0;

let x = 0;
x++, x = 0;
x = 3;

What did you expect to happen? It should mark the last occurance of x as an unused variable.

What actually happened? Please copy-paste the actual, raw output from ESLint. eslint thinks everything is fine.

Steps to reproduce this issue:

1. eslint demo

Are you willing to submit a pull request to fix this bug? No

[snitin315]

I will take this issue 👍🏻

[nzakas]

This is occurring because of the x++, not because of the comma operator. The post fix increment operation counts as both a read and a write, which is why there is no warning.

This looks like a bug but I’d like another set of eyes. @eslint/eslint-tsc