JavaScript类型转换深度学习

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

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

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

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

总结

其它的转换原则还有很多，看到这我们还是不能解释文章开始的代码转换的过程，掌握这些更多是保证正常书写代码规避错误的发生，十分好奇的可以研究下比较特殊的转化原则，还有好多好多。