2020-12-29 18:18:08 +08:00
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# 3变量
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2020-12-16 15:58:59 +08:00
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> javascript的变量是不区分类型的
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## 创建变量
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`var 变量名;`
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`var 变量名 = 变量值;`
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没有值的时候 是创建一个变量 但它的值是`undefined` 这也是默认变量的值 所有没有赋值的变量都是`undefined`
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赋值了之后就变成你赋值之后的值；
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> 实际演示 并且介绍console.log()
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## 变量类型
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2020-12-18 10:55:06 +08:00
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- undefined
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- boolean
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- string
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- number
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- object
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- function
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## typeof
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## undefined
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任何只声明没有赋值的变量都是undefined
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## null
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空对象指针
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## boolean
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只有 true 和 false
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## number
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- 整数 121212
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- 小数 1.12121
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- 八进制 012475
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- 十六进制 0x2474ab
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> 浮点数精度问题
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### NAN
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非常特殊的一个值 应该有值返回的时候但没有值返回 注意 ： nan ！= nan
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### parseInt
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转化到数字
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> 注意8进制
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## string
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- 用‘’ 或 “” 定义
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- 单引号开头必须单引号结尾
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- 双引号开头必须双引号结尾
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### 字符字面量 了解就行
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- \n
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- \t
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- \b
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- \r
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- \f
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- \\\
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- \\'
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- \\"
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- \\xnn
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- \\unnnn
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2020-12-21 09:26:16 +08:00
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### .length
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### .toString()
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## object
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### 创建
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2020-12-21 10:33:16 +08:00
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`new Object() 或者 {}`
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## ES6新增
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- let 变量
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- const 常量
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