> For the complete documentation index, see [llms.txt](https://heihei12305.gitbook.io/my-underscore/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://heihei12305.gitbook.io/my-underscore/han-shu-pan-duan-yu-js-zhong-de-+-jia-zhi.md).

# 函数判断与Js中的’&& ||‘+假值

```
 _.isFunction = function(obj){
    return typeof obj == 'function' || false;
}
```

就是简单判断一个函数而已，不过可以扩展出一个知识点嘿嘿，<br>

你是怎么看js中的`&&,||`？<br>

与其说是逻辑运算符，不如说是属性选择符。

* `&&`执行方式（a&\&b）：
  * 如果a为true，返回b
  * 否则返回a
* `||`执行方式（a || b）:
  * 如果a为true, 返回a
  * 否则返回b

```
'' || [1,2] //[1,2]
'' && [1,2] //""

[1] || [] //[1]
[1] && [] //[]
```

然后把js中为false的属性整理一下啦：&#x20;

\
&#x20;ES5规范9.2定义了抽象操作ToBoolean,列举了布尔强制类型转换所有可能出现的结果。

* undefined
* null
* false
* +0,-0 和 NaN
* ""

从逻辑上说，假值列表以外的都应该是真值。<br>

* 除了'',其他的字符串都是真值。

```
let a  = 'false';
let b = '0';
let c = "''";

let d = Boolean(a && b && c );
d; // true
```

* `[],{},fucntion(){}`都不在假值列表里，所以他们都是真值。

```
let a  = [];
let b = {};
let c = function(){};

let d = Boolean(a && b && c );
d; // true
```

真值列表是无限长的，我们只能根据假值列表作为参考，可以理解位假值列表以外的值都是真值。

* 常用的 || 用法：设置默认值

```
function foo(a,b){
    a = a || 'hello';
    b = b || 'world';

    console.log(a + ' ' + b);
};

foo(1);// '1 world'
```

* js代码压缩工具常用的‘守护运算符’：

```
function foo(){
    console.log(a);
};

let a = 1;
a && foo(); 
//当第一个操作数为真值时，&&才会选择第二个操作数作为返回值，即前面的表达式为后面的表达式'把关'
```


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