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.