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

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

The --fix option on the command line can automatically fix some of the problems reported by 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 Relational Operators.

Relational Operators are:

• 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
}

if(("" + !key) in object) {
    // make operator precedence and type conversion explicit
    // in a rare situation when that is the intended meaning
}

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.

Resources

• Rule source
• Documentation source