> 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-trap/lian-deng-fu-zhi.md).

# 连等赋值

## 引子

```javascript
function foo() {
  let a = b = 0;
  a++;
  return a;
}

foo();
typeof a; // => ???
typeof b; // => ???
```

## 解析

这个代码的重点在第二行:`let a = b = 0`。这个语句声明了一个局部变量 `a`，但是它也声明了一个全局变量`b`。

在 `foo()` 作用域或全局作用域中都没有声明变量 `b`。因此 JS 引荐将`b = 0` 表达式解释为 `window.b = 0`。

如下图所示，函数 `foo` 中的 `i` 都是一个偶然创建的全局变量：

同样，在咱们的问题中，`b` 是一个偶然创建的全局变量。在浏览器中，上面的代码相当于如下：

```javascript
function foo() {
  let a;
  window.b = 0;
  a = window.b;
  a++;
  return a;
}

foo();
typeof a;        // => 'undefined'
typeof window.b; // => 'number'
```

`typeof a` 是 `'undefined'`。变量 `a` 仅在 `foo()` 作用域中声明，在外部作用域内不可用。

`typeof b` 结果是 `'number'`。`b` 是一个值为 `0` 的全局变量


---

# 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, and the optional `goal` query parameter:

```
GET https://mm.ricky.moe/javascript/js-trap/lian-deng-fu-zhi.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.