空JavaScript阵列的布尔值冲突 [英] Conflicting boolean values of an empty JavaScript array
问题描述
有人可以解释为什么以下两个陈述都评估为 true ?
Can anyone explain why the following two statements both evaluate as true?
[] == false
和
!![]
这个问题纯粹出于好奇,为什么会发生这种情况而不是如何最好地测试数组是否为空。
This question is purely out of curiosity of why this happens and not about how to best test if an array is empty.
推荐答案
第一个:
[] == false
==
运算符对其操作数进行类型转换,在这种情况下,双方都转换为数字,在抽象平等比较算法将是:
The ==
operator does type conversion to its operands, in this case the both sides are converted to Number, the steps taken on the Abstract Equality Comparison Algorithm would be:
- object == boolean
- object == number
- string == number
- number == number
- object == boolean
- object == number
- string == number
- number == number
代码:
[] == false; // convert false to Number
[] == 0; // convert [] to Primitive (toString/valueOf)
"" == 0; // convert "" to Number
0 == 0; // end
第二次比较, []
转换为原始,它们的 valueOf
和 toString
方法被执行,但是因为 valueOf
在Array对象上,返回对象本身(继承自 Object.prototype
),然后 toString
使用方法。
The second comparison, []
is converted to primitive, their valueOf
and toString
methods are executed, but since valueOf
on Array objects, returns the object itself (is inherited from Object.prototype
), then the toString
method is used.
如您所见,两个操作数都转换为Number,两者都为零,例如:
At the end as you see, both operands are converted to Number, and both yield zero, for example:
Number([]) == 0;
Number(false) == 0;
当转换为Number时,空数组产生零,因为其字符串表示形式为空字符串:
And empty array produces zero when converted to Number because its string representation is an empty string:
[].toString(); // ""
空字符串转换为数字,产生零:
And an empty string converted to Number, yields zero:
+""; // 0
现在,双重否定( !! []
)生成true,因为所有对象实例都是真实的:
Now, the double negation (!![]
) produces true because all object instances are truthy:
![]; // false, [] is truthy
!![]; // true, negation
falsey 的唯一值是:
-
null
-
未定义
-
0
-
NaN
-
(空字符串)
-
false
null
undefined
0
NaN
""
(an empty string)false
其他任何东西都会产生 true
转换为布尔值时。
Anything else will produce true
when converted to Boolean.
参见:
- JavaScript Coercion Tool
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