no-unsafe-negation.md

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

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:

• in operator.
• instanceof operator.

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

The enforceForOrderingRelations option extends the rule to apply to all Relational Operators, not just in and instanceof.

With this option set to true, the rule is enforced for all of the following:

• in operator.
• instanceof operator.
• < 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-unneeded-ternary: ["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.