Check `!foo === bar` in `no-unsafe-negation` rule

What rule do you want to change?

Forbid use !foo === bar and !foo !== bar

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

more

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

Not sure

Please provide some example code that this change will affect:

if (!foo === bar);

What does the rule currently do for this code?

Nothing

What will the rule do after it’s changed?

Forbid it.

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

Maybe.

---

I understand !foo === bar maybe just coverting falsely value to compare to a boolean, but most time it’s a mistake,

I guess we should allow

// it's type-casting
if (!!foo === bar);

if (!(foo === bar));

if ((!foo) === bar);

I understand that stylistic rules are frozen, but still worth to make a proposal. I can understand if you decide not to accept.

[mdjermanovic]

Looking at this more closely, I’m not sure adding checks for equality operators make sense. The point of this rule is to warn when negating a value will cause an unexpected result. It makes sense for the “in” and “instanceof” operators because you likely mistyped. It makes sense for comparison operators because you can’t really compare Boolean values.

I agree with @nzakas on this. !a in b or !a < b almost never make sense, but !a === b is valid code if b is a boolean. Since we don’t have type info, an error reported on such code would be a false positive, or could be interpreted as stylistic enforcement which isn’t the intent of this rule.

I was in favor of this option in no-unsafe-negation, but now I think that we should report !a === b as a potential problem only if we’re sure that the right side is not a boolean, so this rather belongs to no-constant-binary-expression rule (proposal https://github.com/eslint/eslint/issues/13752, there’s !foo == null example in the original post), which will be based on determining types of the operands when possible. That would catch https://github.com/prettier/prettier/commit/c7e354e38cfd051d3f64b56aeda35e9a0a669de6#r47946398 as <boolean> === <string>.

[fisker]

@coderaiser Stop, please!