function() { var c; c == 0; } fires the "useless expression" warning, but function() { c == 0; } does not. I think the latter should as well, because even though the subexpression c is useful, the comparison is useless. Bug 382584 contains an example of a bug where an assignment (=) was mistyped as comparison (==). In that instance, the LHS was a local variable, so the warning was able to fire. But if the LHS had been a global variable or a property of something, the existing warning would not have fired.

There's a difference between function() { var c; c == 0; } and function() { c == 0; } The second one sets/changes a global variable called 'c', hence it's not a "useless" expression. Working as per Javascript specification. Therefore I suggest to close this bug.

Indeed, my mistake (I tohught it was c=0;}. c==0 is indeed a useless expression.

However, if 'c' variable is defined through a getter, then the getter could have side effects. Then that call would not be useless.

Any local variable access is nowadays considered an effectful expression (see bug 1167823), therefore resolving as WONTFIX.