是！！ Javascript 中是否需要运算符？

发布于 2024-12-11 23:11:11 字数 313 浏览 0 评论 0原文

我可以理解当您想要将对象值转换为布尔值并将其保存在变量中的情况。然而，我在 jQuery 模板中遇到了以下代码，并且想知道 !! （双感叹号运算符）甚至是必要的。

{{if !!sectionId}}
    // do something...
{{/if}}

我假设不是，因为 Javascript 会自动将 if 后面的表达式计算为布尔值。因此，你可以写：

{{if sectionId}}
    // do something...
{{/if}}

我的假设正确吗？

原文

I can understand cases when you will want to convert an object value to a boolean and save it in a variable. However, I came across the following code in a jQuery template and was wondering if the !! (double exclamation operators) is even necessary.

{{if !!sectionId}}
    // do something...
{{/if}}

I am assuming that it is not since Javascript will automatically evaluate the expression following the if as boolean. Therefore, you could just write:

{{if sectionId}}
    // do something...
{{/if}}

Am I right in my assumption?

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赴月观长安 2024-12-18 23:11:11

JavaScript 中没有 !! 运算符。只有 !。您所看到的是该单个运算符的双重应用程序。

! 的单个应用程序将通过评估其参数的“真实性”来返回一个布尔值，并给出该值的布尔值倒数。因此，第二个 ! 给出了那个值的布尔逆，这就是原始值的布尔“真实性”。

就我个人而言，我不会像您的示例一样在简单的 if 语句中使用它，但它对于可能显式检查布尔类型参数的 API 来说很方便：

someAPI( !! someExpression );

There is no !! operator in JavaScript. There's just !. What you're seeing is a doubled application of that single operator.

A single application of ! will return a boolean by evaluating the "truthiness" of its argument, giving the boolean inverse of that. The second ! therefore gives the boolean inverse of that value, which is thus the boolean "truthiness" of the original value.

Personally I wouldn't use it in a simple if statement as in your example, but it's handy for APIs that might explicitly check for a boolean-typed parameter: