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
}

Options

Nothing.

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