JS.BASE.NO.NEGATED.IN.LHS

Disallow negating the left operand in 'in' expressions

This rule was deprecated in ESLint v3.3.0 and replaced by the no-unsafe-negation 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.

Rule Details

This rule disallows negating the left operand in in expressions.

Examples of incorrect code for this rule:

Copy

/*eslint no-negated-in-lhs: "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
}

Examples of correct code for this rule:

Copy

/*eslint no-negated-in-lhs: "error"*/

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

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

Never.