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JavaScript 类型转换深度学习

JavaScript 类型转换深度学习

JavaScript 是一门弱类型语言,刚接触的时候感觉方便快捷(不需要声明变量类型了耶!),接触久了会发现它带来的麻烦有的时候不在预期之内

呵呵一笑,哪有这么夸张,可能有人看过这样一段代码

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这个占了好大的篇幅哈 3167 个字符,粘贴到浏览器的 Console 控制台,直接弹出了 orange,随叫随到有不有

对于不知道原理出处的给大家一个地址: http://www.jsfuck.com/

JSFuck 的变态程度达到了极致,因为它的理念是 Write any JavaScript with 6 Characters: []()!+

或许又有人说:这个只是搞怪的吧,实际谁这么写代码啊

说的没错,当一段代码变得晦涩难懂的时候,甚至到上文的混乱字符(天书),却能实现任意功能这就变得不可预期,也就是说 JS 代码的安全性没有保障

当然本文不会研究这些无意义的字符原理是怎么实现的因为人家的 Github 文档已经描述的特别全面了,感兴趣的可以研究下: https://github.com/aemkei/jsfuck

我们聊一聊每天能看到用到的方法底层是怎么解析的,熟知转换分成两种一种是隐式转换,另一种是强制的类型转换

隐式转换

当遇到以下几种情况,JavaScript会自动转换数据类型:

  • 不同类型的数据进行互相运算
  • 对非布尔值类型的数据求布尔值
  • 对非数值类型的数据使用一元运算符(即 “+” 和 “-“)

隐式转换为 Boolean

大多数在做 if 判断时会用到,这里只需记住六个转换为 false,其它全部为 true

  • null
  • undefined
  • NaN
  • ‘’
  • -0
  • +0

隐式转换为 String

字符串的自动转换,主要发生在加法运算时。当一个值为字符串,另一个值为非字符串,则后者转为字符串。

'1' + 2  // '12'
'1' + true  // "1true"
'1' + false  // "1false"
'1' + {}  // "1[object Object]"
'1' + []  // "1"
'1' + function (){}  // "1function (){}"
'1' + undefined  // "1undefined"
'1' + null  // "1null"

隐式转换为 Number

除了加法运算符有可能把运算子转为字符串,其他运算符都会把两侧的运算子自动转成数值

'5' - '2'  // 3
'5' * '2'  // 10
true - 1  // 0
false - 1  // -1
'1' - 1  // 0
'5' * []  // 0
false / '5'  // 0
'abc' - 1  // NaN
+'abc'  // NaN
-'abc'  // NaN
+true  // 1
-false  // 0

隐式转换的基础表现都在这了,强调的是这些转换的背后都伴随着强制转换,使用 Boolean、Number 和 String,下面重点讲一下强制转换的原理

强制转换

看到上面例子也许你已经有些许疑问了,比如上面的这个 '1' + {} 怎么就输出 1[object Object] 了呢

如上面强调的,你会猜测首先执行 String({}) 得到 "[object Object]" ,然后再字符串拼接,是的我们总能得到转换背后的实现原理,其实真实原理要比这个复杂,见下文

强制转换为 Boolean

这里略过因为与隐式转换相同,切记 []、{} 都转换成 true

强制转换为 String

基本类型的转换结果与隐式转换相同,这里说一下对象的转换,加深上面引用例子的解析

对象转换字符串分成三步

  1. 先调用toString方法,如果toString方法返回的是原始类型的值,则对该值使用String方法,不再进行以下步骤
  2. 如果toString方法返回的是复合类型的值,再调用valueOf方法,如果valueOf方法返回的是原始类型的值,则对该值使用String方法,不再进行以下步骤
  3. 如果valueOf方法返回的是复合类型的值,则报错

再分解这个例子

String({})
// "[object Object]"

上面代码相当于下面这样

String({}.toString())
// "[object Object]"

如果 toString 方法和 valueOf 方法,返回的都不是原始类型的值,则 String 方法报错

var obj = {
  valueOf: function () {
    console.log("valueOf");
    return {};
  },
  toString: function () {
    console.log("toString");
    return {};
  }
};

String(obj)
// TypeError: Cannot convert object to primitive value

我们不难看出可以对 toString 方法和 valueOf 方法进行改写,测试其先后运行的顺序也简单的多

String({toString:function(){return 3;}})
// "3"

String({valueOf:function (){return 2;}})
// "[object Object]"

String({valueOf:function (){return 2;},toString:function(){return 3;}})
// "3"

结果表示toString方法先于valueOf方法执行

强制转换为 Number

基本类型转换如下

Number("123") // 123

Number("123abc") // NaN

Number("") // 0

Number(false) // 0

Number(undefined) // NaN

Number(null) // 0

对象转换一样要复杂些,与 String 唯一不同的就是 valueOf 方法在前, toString 方法在后,其它不赘述见上文例子。

isNaN() 并不陌生, isNaN({}) //true 的内在转换过程是相同的

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