79 lines
2.0 KiB
Markdown
79 lines
2.0 KiB
Markdown
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---
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title: Numbers
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localeTitle: 数字
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---
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## 数字
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JavaScript `number`的实现基于`IEEE 754`标准，通常称为“浮点”。
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[IEEE 754维基百科链接](https://en.wikipedia.org/wiki/IEEE_754) [IEEE 754双精度浮点可视化](http://bartaz.github.io/ieee754-visualization/)
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数字文字通常表示为`base-10`十进制文字。
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```javascript
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var foo = 47;
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var bar = 47.9;
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```
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十进制值的前导部分（如果为`0` ）是可选的：
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```javascript
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var same = 0.47;
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var stillSame = .47;
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```
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同样，后面的十进制值的尾部（小数） `.` ，如果为`0` ，则是可选的：
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```javascript
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var a = 47.0;
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var b = 47.;
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```
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默认情况下，大多数数字将输出为`base-10`小数，并删除尾随小数`0` 。所以：
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```javascript
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var foo = 47.300;
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var bar = 47.0;
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foo; // 47.3
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bar; // 47
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```
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非常大或非常小的`numbers`可写为：
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```javascript
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var foo = 47e8; // 4700000000
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var baz = 47e-8; // 00.00000047
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```
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`toExponential`方法可用于将`number`转换为`exponential notation` 。
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```javascript
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var foo = 47e8;
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foo; // 4700000000
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foo.toExponential() //"47e8"
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```
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Numbers可以访问`Number.prototype`中内置的方法。
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例如： `toFixed()`方法格式化一个具有小数点右侧特定位数的数字。
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```javascript
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var foo = 47.69;
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foo.toFixed(0); // "48"
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foo.toFixed(1); // "47.7"
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foo.toFixed(2); // "47.69"
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```
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> 在浏览器中键入`Number.prototype`并自己查看其他可用方法。
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#### 更多信息：
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1. [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type)
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2. [JavaScript编号](https://www.w3schools.com/js/js_numbers.asp)
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#### 参考
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1. Kyle Simpson的[类型和语法](https://github.com/getify/You-Dont-Know-JS/tree/master/types%20%26%20grammar) 。
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2. [ECMAScript语言规范：4.3.20](https://www.ecma-international.org/ecma-262/5.1/#sec-4.3.20)
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3. [ECMAScript语言规范：15.7数字对象](https://www.ecma-international.org/ecma-262/5.1/#sec-15.7)
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