no-unsafe-negation

Disallow negating the left operand of relational operators

就像开发者可能会输入 -a + b，而他们的意思是 -(a + b) 表示一个和的负数，他们可能会错误地输入 ！key in object，而他们几乎肯定是指 !(key in object) 来测试一个键不在一个对象中。!obj instanceof Ctor 是类似的。

规则细节

这条规则不允许否定以下关系运算符的左边操作数。

• in 运算符.
• instanceof 运算符.

使用此规则的错误示例：

/*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.
}
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使用此规则的正确示例：

/*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
}
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Exception

对于极少数打算否定左边操作数的情况，本规则允许有例外。 如果整个否定被明确地包裹在括号中，本规则将不会报告问题。

使用此规则的正确示例：

/*eslint no-unsafe-negation: "error"*/

if ((!foo) in object) {
    // allowed, because the negation is explicitly wrapped in parentheses
    // it is equivalent to (foo ? "false" : "true") in object
    // this is allowed as an exception for rare situations when that is the intended meaning
}

if(("" + !foo) in object) {
    // you can also make the intention more explicit, with type conversion
}
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使用此规则的错误示例：

/*eslint no-unsafe-negation: "error"*/

if (!(foo) in object) {
    // this is not an allowed exception
}
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选项

此规则选项为对象：

• "enforceForOrderingRelations": false（默认值）允许否定排序关系运算符的左侧（<, >, <=, >=）。
• "enforceForOrderingRelations": true 不允许对排序关系运算符的左侧进行否定。

enforceForOrderingRelations

如果该选项设置为 true，该规则将单独强制执行：

• < operator.
• > operator.
• <= operator.
• >= operator.

其目的是为了避免诸如 !a < b（相当于 (a ? 0 : 1) < b）的表达，而真正的目的是 !(a < b)。

使用此规则与 { "enforceForOrderingRelations": true } 选项的额外错误示例：

/*eslint no-unsafe-negation: ["error", { "enforceForOrderingRelations": true }]*/

if (! a < b) {}

while (! a > b) {}

foo = ! a <= b;

foo = ! a >= b;
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何时不用

如果你不想通知不安全的逻辑否定句，那么你可以安全地禁用此规则。

Handled by TypeScript

It is safe to disable this rule when using TypeScript because TypeScript's compiler enforces this check.

Version

This rule was introduced in ESLint v3.3.0.

Resources

• Rule source
• Tests source