75 lines
1.9 KiB
Markdown
75 lines
1.9 KiB
Markdown
---
|
||
title: Falsy Values
|
||
localeTitle: 虚假的价值观
|
||
---
|
||
## 描述
|
||
|
||
虚假值是评估为FALSE的值，例如在检查变量时。 JavaScript中只有六个falsey值： `undefined` ， `null` ， `NaN` ， `0` ， `""` （空字符串），当然还有`false` 。
|
||
|
||
## 检查变量上的假值
|
||
|
||
可以使用简单的条件检查变量中的假值：
|
||
|
||
```javascript
|
||
if (!variable) {
|
||
// When the variable has a falsy value the condition is true.
|
||
}
|
||
```
|
||
|
||
## 一般例子
|
||
|
||
```javascript
|
||
const string = ""; // <-- falsy
|
||
|
||
const filledString = "some string in here"; // <-- truthy
|
||
|
||
const zero = 0; // <-- falsy
|
||
|
||
let numberGreaterThanZero; // <-- falsy
|
||
|
||
const emptyArray = []; // <-- truthy, we'll explore more about this next
|
||
|
||
const emptyObject = {}; // <-- truthy
|
||
```
|
||
|
||
## 有阵列的乐趣
|
||
|
||
```javascript
|
||
if ([] == false) // <-- truthy, will run code in if-block
|
||
|
||
if ([]) // <-- truthy, will also run code in if-block
|
||
|
||
if ([] == true) // <-- falsy, will NOT run code in if-block
|
||
|
||
if (![]) // <-- falsy, will also NOT run code in if-block
|
||
```
|
||
|
||
## 警告
|
||
|
||
在布尔上下文中评估值时，请注意数据类型。如果值的数据类型是一个_数字_ ，那么truthy / falsy评估可能会导致意外的结果：
|
||
|
||
```javascript
|
||
const match = { teamA: 0, teamB: 1 }
|
||
if (match.teamA)
|
||
// The following won't run due to the falsy evaluation
|
||
console.log('Team A: ' + match.teamA);
|
||
}
|
||
```
|
||
|
||
上面用例的替代方法是使用`typeof`评估值：
|
||
|
||
```javascript
|
||
const match = { teamA: 0, teamB: 1 }
|
||
if (typeof match.teamA === 'number')
|
||
console.log('Team A: ' + match.teamA);
|
||
}
|
||
```
|
||
|
||
## 更多信息
|
||
|
||
* **真实的** [博客文章 - Truthy＆Falsey](http://james.padolsey.com/javascript/truthy-falsey/)
|
||
|
||
* [Falsy |词汇表| MDN](https://developer.mozilla.org/en-US/docs/Glossary/Falsy)
|
||
|
||
* [Truthy和Falsy：当JavaScript中的所有不相等时](https://www.sitepoint.com/javascript-truthy-falsy/)
|