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Description
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:
inoperator.instanceofoperator.
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
}http://eslint.org/docs/rules/no-unsafe-negation
Likely to catch bugs, and there's basically never a reason to write code that this would flag, so I'll merge this into the standard v8 beta.
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