74 lines
1.9 KiB
Markdown
74 lines
1.9 KiB
Markdown
---
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title: Falsy Values
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localeTitle: 虚假的价值观
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---
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## 描述
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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` 。
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## 检查变量上的假值
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可以使用简单的条件检查变量中的假值：
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```javascript
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if (!variable) {
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// When the variable has a falsy value the condition is true.
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}
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```
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## 一般例子
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```javascript
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var string = ""; // <-- falsy
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var filledString = "some string in here"; // <-- truthy
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var zero = 0; // <-- falsy
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var numberGreaterThanZero // <-- truthy
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var emptyArray = []; // <-- truthy, we'll explore more about this next
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var emptyObject = {}; // <-- truthy
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```
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## 有阵列的乐趣
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```javascript
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if ([] == false) // <-- truthy, will run code in if-block
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if ([]) // <-- truthy, will also run code in if-block
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if ([] == true) // <-- falsy, will NOT run code in if-block
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if (![]) // <-- falsy, will also NOT run code in if-block
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```
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## 警告
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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：
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```javascript
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const match = { teamA: 0, teamB: 1 }
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if (match.teamA)
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// The following won't run due to the falsy evaluation
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console.log('Team A: ' + match.teamA);
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}
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```
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上面用例的替代方法是使用`typeof`评估值：
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```javascript
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const match = { teamA: 0, teamB: 1 }
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if (typeof match.teamA === 'number')
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console.log('Team A: ' + match.teamA);
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}
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```
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## 更多信息
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* **真实的** [博客文章 - Truthy＆Falsey](http://james.padolsey.com/javascript/truthy-falsey/)
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* [Falsy |词汇表| MDN](https://developer.mozilla.org/en-US/docs/Glossary/Falsy)
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* [Truthy和Falsy：当JavaScript中的所有不相等时](https://www.sitepoint.com/javascript-truthy-falsy/) |