disallow negating the left operand of relational operators (no-unsafe-negation)

The "extends": "eslint:recommended" property in a configuration file enables this rule.

Just as developers might type -a + b when they mean -(a + b) for the negative of a sum, they might type !key in object by mistake when they almost certainly mean !(key in object) to test that a key is not in an object. !obj instanceof Ctor is similar.

Rule Details

This rule disallows negating the left operand of the following relational operators:

Examples of incorrect code for this rule:

/*eslint no-unsafe-negation: "error"*/

if (!key in object) {
    // operator precedence makes it equivalent to (!key) in object
    // and type conversion makes it equivalent to (key ? "false" : "true") in object
}

if (!obj instanceof Ctor) {
    // operator precedence makes it equivalent to (!obj) instanceof Ctor
    // and it equivalent to always false since boolean values are not objects.
}

Examples of correct code for this rule:

/*eslint no-unsafe-negation: "error"*/

if (!(key in object)) {
    // key is not in object
}

if (!(obj instanceof Ctor)) {
    // obj is not an instance of Ctor
}

Exception

For rare situations when negating the left operand is intended, this rule allows an exception. If the whole negation is explicitly wrapped in parentheses, the rule will not report a problem.

Examples of correct code for this rule:

/*eslint no-unsafe-negation: "error"*/

if ((!foo) in object) {
    // allowed, because the negation is explicitly wrapped in parentheses
    // it is equivalent to (foo ? "false" : "true") in object
    // this is allowed as an exception for rare situations when that is the intended meaning
}

if(("" + !foo) in object) {
    // you can also make the intention more explicit, with type conversion
}

Examples of incorrect code for this rule:

/*eslint no-unsafe-negation: "error"*/

if (!(foo) in object) {
    // this is not an allowed exception
}

Options

This rule has an object option:

"enforceForOrderingRelations": false (default) allows negation of the left-hand side of ordering relational operators (<, >, <=, >=)
"enforceForOrderingRelations": true disallows negation of the left-hand side of ordering relational operators

enforceForOrderingRelations

With this option set to true the rule is additionally enforced for:

< operator.
> operator.
<= operator.
>= operator.

The purpose is to avoid expressions such as ! a < b (which is equivalent to (a ? 0 : 1) < b) when what is really intended is !(a < b).

Examples of additional incorrect code for this rule with the { "enforceForOrderingRelations": true } option:

/*eslint no-unsafe-negation: ["error", { "enforceForOrderingRelations": true }]*/

if (! a < b) {}

while (! a > b) {}

foo = ! a <= b;

foo = ! a >= b;

When Not To Use It

If you don't want to notify unsafe logical negations, then it's safe to disable this rule.

Version

This rule was introduced in ESLint 3.3.0.