Question

假设我想转换一个显然不是数字的字符串，但是在最后我会得到一个数字！？

var b = parseFloat("a");

console.log(b); // output NaN

console.log(typeof b); // print number!

这到底发生了什么？ NaN意味着不是数字，对吧？

好的，所以NaN == NaN总是假的。但是：

typeof 1/0 // prints NaN
1/0 == 1/0 // its true

Answer 1

它并不特定于Javascript。它与计算机科学更相关。

在计算中，NaN（不是数字）是表示未定义或不可表示值的数值数据类型值，尤其是在浮点计算中。

有三种操作可以返回NaN

Operations with a NaN as at least one operand.
Indeterminate forms
    The divisions 0/0 and ±∞/±∞
    The multiplications 0×±∞ and ±∞×0
    The additions ∞ + (−∞), (−∞) + ∞ and equivalent subtractions
    The standard has alternative functions for powers:
        The standard pow function and the integer exponent pown function define 00, 1∞, and ∞0 as 1.
        The powr function defines all three indeterminate forms as invalid operations and so returns NaN.

Real operations with complex results, for example:
    The square root of a negative number.
    The logarithm of a negative number
    The inverse sine or cosine of a number that is less than −1 or greater than +1.

Answer 2

var b = parseFloat("a");

console.log(b); // output NaN

console.log(typeof b);  // number! what else would it be?
console.log(!isNaN(b)); // false 
console.log(isNaN(b));  // true!

console.log(isNaN('stringoftext'));  // true! this is not good, that's REALLY not a number
//https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

console.log(typeof b === 'number' && isNaN(b) === true); // It is NOT A NUMBER

console.log(b != b); // true , means that it is NaN
//http://stackoverflow.com/a/14220688/3741423

Answer 3

Threre不是你想要的解释。功能parseFloat可能会引发异常，但这是另一种情况。它返回特殊的＆＃34;数字＆＃34;叫＆＃34;不是号码＆＃34;。只是不要带着这个名字循环。

为什么不是数字（NaN）是Javascript中的数字？

3 个答案: