javascript 是 ！！在 if 语句中检查真值的最佳实践
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Is !! a best practice to check a truthy value in an if statement
提问by Bargitta
In angular.js, there are some code snippets use !!to check whether a value is truthy in if condition.
在 angular.js 中，有一些代码片段用于!!检查 if 条件下的值是否为真。
Is it a best practice? I fully understand in return value or other assignment !! is used to make sure the type is Boolean. But is it also true for condition checks?
这是最佳做法吗？我完全理解返回值或其他赋值！！用于确保类型为布尔值。但条件检查也是如此吗？
if (!!value) {
element[name] = true;
element.setAttribute(name, lowercasedName);
} else {
element[name] = false;
element.removeAttribute(lowercasedName);
}回答by Denys Séguret
No, !!is totally useless in a ifcondition and only confuses the reader.
不，!!在某种if情况下完全没有用，只会让读者感到困惑。
Values which are translated to truein !!valuealso pass the iftest because they're the values that are evaluated to truein a Boolean context, they're called "truthy".
转换为truein 的值!!value也通过if测试，因为它们是true在布尔上下文中计算的值，它们被称为"truthy"。
So just use
所以只需使用
if (value) {
回答by Timothy Shields
!!valueis commonly used as a way to coerce valueto be either trueor false, depending on whether it is truthy or falsey, respectively.
!!value通常用作强制value为true或 的一种方式false，分别取决于它是真还是假。
In a control flow statement such as if (value) { ... }or while (value) { ... }, prefixing valuewith !!has no effect, because the control flow statement is already, by definition, coercing the valueto be either trueor false. The same goes for the condition in a ternary operator expression value ? a : b.
在诸如if (value) { ... }or 之类的控制流语句中while (value) { ... }，前缀valuewith!!无效，因为根据定义，控制流语句已经将 强制value为trueor 或false。三元运算符表达式中的条件也是如此value ? a : b。
Using !!valueto coerce valueto trueor falseis idiomatic, but should of course only be done when it isn't made redundant by the accompanying language construct.
使用!!valueto coerce valuetotrue或false是惯用的，但当然应该只在它没有被附带的语言结构变得多余时才使用。
