39 lines
1.0 KiB
Markdown
39 lines
1.0 KiB
Markdown
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---
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title: Function Length
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localeTitle: 功能长度
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---
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## 功能长度
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函数对象的`length`属性保存调用时函数所期望的参数数。
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```javascript
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function noArgs() { }
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function oneArg(a) { }
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console.log(noArgs.length); // 0
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console.log(oneArg.length); // 1
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```
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### ES2015语法
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ES2015，或通常称为ES6，引入了rest操作符和默认函数参数。这两个添加都改变了`length`属性的工作方式。
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如果在函数声明中使用了rest运算符或默认参数，则`length`属性将仅包含rest运算符或默认参数之前的参数数。
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```javascript
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function withRest(...args) { }
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function withArgsAndRest(a, b, ...args) { }
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function withDefaults(a, b = 'I am the default') { }
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console.log(withRest.length); // 0
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console.log(withArgsAndRest.length); // 2
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console.log(withDefaults.length); // 1
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```
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有关`Function.length`更多信息可以在[Mozilla的MDN Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/length)上找到。
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