判断揭秘

作者: hhuwc | 来源:发表于2017-12-12 16:55 被阅读0次

也许你运行代码时会感到困惑，为什么字符串可以作为判断条件，字符串和true 进行运算结果会是true

if ("potato") {
    console.log("potato" == false); //false
    console.log("potato" == true); //false
}

在JavaScript中，所有的条件语句和操作符遵循相同的强制转换规范。比如说if（Expression）语句将使用抽象方法ToBoolean将表达式求值的结果强制为布尔值，为此，ES5规范定义了以下算法：

Argument Type	Result
Undefined	false
Null	false
Boolean	The result equals the input argument (no conversion).
Number	The result is false if the argument is +0, −0, or NaN;otherwise the result is true.
String	The result is false if the argument is the empty String (its length is zero);otherwise the result is true.
Object	true

所以 "potato" 作为表达式转换结果将是 true

==那么 "potato" == false是怎么进行运算的，如果"potato" == 1呢？==

运算规则如下：

Type(x)	Type(y)	Result
x and y are the same type		See Strict Equality (===) Algorithm
null	Undefined	true
Undefined	null	true
Number	String	x == toNumber(y)
String	Number	toNumber(x) == y
Boolean	(any)	toNumber(x) == y
(any)	Boolean	x == toNumber(y)
String or Number	Object	x == toPrimitive(y)
Object	String or Number	toPrimitive(x) == y
otherwise…		false

那么toNumber ,toPrimitive 是按照什么规则进行转换的

ToNumber

Argument Type	Result
Undefined	NaN
Null	+0
Boolean	The result is 1 if the argument is true.The result is +0 if the argument is false.
Number	The result equals the input argument (no conversion).
String	In effect evaluates Number(string);“abc” -> NaN;“123” -> 123
Object	Apply the following steps:1. Let primValue be ToPrimitive(input argument, hint Number).2. Return ToNumber(primValue).

ToPrimitive

Argument Type	Result
Object	(in the case of equality operator coercion) if valueOf returns a primitive, return it. Otherwise if toString returns a primitive return it. Otherwise throw an error
otherwise…	The result equals the input argument (no conversion).