> For the complete documentation index, see [llms.txt](https://mm.ricky.moe/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mm.ricky.moe/javascript/js-concept/undefined.md).

# !!

有的地方能看到`!!` 这样的操作符，实际上这是`!` 的两次使用。

其真实目的是把一个非`boolean`变量转换为`boolean` 变量, 还有这样写法比较简洁。

```javascript
!!false === false
!!true === true

!!0 === false
!!parseInt("foo") === false // NaN is falsy
!!1 === true
!!-1 === true  // -1 is truthy

!!"" === false // empty string is falsy
!!"foo" === true  // non-empty string is truthy
!!"false" === true  // ...even if it contains a falsy value

!!window.foo === false // undefined is falsy
!!null === false // null is falsy

!!{} === true  // an (empty) object is truthy
!![] === true  // an (empty) array is truthy; PHP programmers beware!
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://mm.ricky.moe/javascript/js-concept/undefined.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.