为什么 JavaScript 说数字不是数字？
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Why JavaScript says that a number is not a number?
提问by Arseni Mourzenko
I have a piece of JavaScript code which is expected to set an integer value to a variable.
我有一段 JavaScript 代码，预计将一个整数值设置为一个变量。
Something is broken, so when I try to do alert(A);, it returns NaN. isNaN(A);returns true. But if I alert(typeof(A));, it says number.
有些东西坏了，所以当我尝试做时alert(A);，它会返回NaN。isNaN(A);返回真。但如果我alert(typeof(A));，它说number。
So how can a variable be a number and not a number at the same time? Maybe I misunderstood what NaN really is?
那么一个变量怎么能同时是一个数字而不是一个数字呢？也许我误解了 NaN 到底是什么？
Edit:thanks to the answers, I see that I was wrong, because:
编辑：感谢答案，我发现我错了，因为：
- The type of
NaNisNumber, NaNdoes mean "Not a number", which is not the same thing as "not of typeNumber",0/0is a good example ofNaN: it is still a number, but JavaScript (and nobody else) can say what is the real value of zero divided by zero.1/0on the other hand returnsInfinity, which is notNaN.
- 的类型
NaN是Number， NaN确实意味着“不是数字”，这与“非类型Number”不同，0/0是一个很好的例子NaN：它仍然是一个数字，但 JavaScript（和其他人）可以说出零除以零的实际值是多少。1/0另一方面返回Infinity，这不是NaN。
采纳答案by Andrzej Doyle
As I understand it, NaNis a sentinel instance of the Numberclass that represents, well, exactly what it stands for - numeric results that cannot be adequately represented. So 0/0is not a number, in the sense that it's NaN, but it isa Numberin terms of its type.
据我了解，它NaN是Number该类的一个哨兵实例，它代表了它所代表的内容 - 无法充分表示的数字结果。So0/0不是数字，就其类型而言，NaN它是，但就Number其类型而言，它是a 。
Perhaps it should have been called NaRN (Not a Representable Number).
也许它应该被称为 NaRN（Not a Representable Number）。
回答by tvanfosson
If you have a variable and assign it the result of 0/0, the variable is still of numeric type, but the value is undefined (not a number). There are other conditions under which this can occur, but this illustrates what you are seeing.
如果您有一个变量并为其分配结果 0/0，则该变量仍然是数字类型，但值未定义（不是数字）。还有其他条件可能会发生这种情况，但这说明了您所看到的情况。
回答by Janick Bernet
You are confusing the typeof your object with the value. NaNis a specific value that a an object of type numbercan be assigned with, for instance in the case of division of zero by zero or when trying to convert a number from a string that does not represent a number.
您将对象的类型与值混淆了。NaN是number可以分配给类型对象的特定值，例如在零除以零的情况下或尝试从不表示数字的字符串转换数字时。
回答by J?rgen Fogh
You should check out the Wikipedia article. It has more details.
您应该查看维基百科文章。它有更多细节。
回答by user347594
Some definitions from W3Schools:
W3Schools 的一些定义：
Infinity: A numeric value that represents positive/negative infinity
Infinity：表示正/负无穷大的数值
The POSITIVE_INFINITY property represents infinity, returned on overflow. NEGATIVE_INFINITY, represents negative infinity (returned on overflow).
POSITIVE_INFINITY 属性表示无穷大，在溢出时返回。NEGATIVE_INFINITY，表示负无穷大（溢出时返回）。
The NaN property represents "Not-a-Number" value. This property indicates that a value is not a legal number.
NaN 属性表示“非数字”值。此属性指示值不是合法数字。
The isFinite() function determines whether a number is a finite, legal number. This function returns false if the value is +infinity, -infinity, or NaN.
isFinite() 函数确定一个数字是否是一个有限的合法数字。如果值为 +infinity、-infinity 或 NaN，则此函数返回 false。
Some tests:
一些测试：
var n1 = 1/0;
var n2 = 0/0;
var n3 = (Number.MAX_VALUE)*2; //overflow
var b1 = Number.POSITIVE_INFINITY == n1;
var b2 = Number.POSITIVE_INFINITY == n2;
var b2n = Number.NEGATIVE_INFINITY == n2;
var b3 = Number.POSITIVE_INFINITY == n3;
var msg = "n1=" + n1 + ", n2=" + n2 + ", n3=" + n3;
msg += "<br/> n1 Is POSITIVE_INFINITY=" + b1;
msg += "<br/> n2 Is POSITIVE_INFINITY=" + b2;
msg += "<br/> n2 Is POSITIVE_INFINITY=" + b2n;
msg += "<br/> n3 Is POSITIVE_INFINITY=" + b3;
msg += "<br/> n1 IsFinite=" + isFinite(n1);
msg += "<br/> n2 IsFinite=" + isFinite(n2);
msg += "<br/> n3 IsFinite=" + isFinite(n3);
msg += "<br/> n1 + n1 =" + (n1 + n1) ;
msg += "<br/> n1 - n1 =" + (n1 - n1) ;
msg += "<br/> n2 + n1 =" + (n2 + n1) ;
document.write(msg);
Shows
节目
n1=Infinity, n2=NaN, n3=Infinity
n1 Is POSITIVE_INFINITY=true
n2 Is POSITIVE_INFINITY=false
n2 Is POSITIVE_INFINITY=false
n3 Is POSITIVE_INFINITY=true
n1 IsFinite=false
n2 IsFinite=false
n3 IsFinite=false
n1 + n1 =Infinity
n1 - n1 =NaN
n1 - n1 =NaN
回答by qw3n
Along with what has been said I think if you divide by for example a string. It trys to returns NaN but still looks at it as a number.
除了上面所说的，我认为如果你除以一个字符串。它尝试返回 NaN 但仍将其视为数字。
