When and why number evaluates to NaN, after multiplying, in Javascript?

问题

From my experience I know, that I can get NaN when doing some division or when trying to make a number out of string, when it actually does not contain a number. Are there any other situations, when I can get a NaN. In particular -- is it possible to get it from multiplication?

This is a piece of code, that I use in my PhoneGap application:

var g = 9.80665;

acceleration.x = (acceleration.x * g).toFixed(2);
acceleration.y = (acceleration.y * g).toFixed(2);
acceleration.z = ((acceleration.z + 1) * g).toFixed(2);

An acceleration is a base PhoneGap object and I can hardly believe, that it contains a string or any other value, except float, that could result in a NaN.

However, in some certain situations (on some specific devices) I'm getting a NaN out of above.

It isn't a problem for me to "secure" above values with a code like this:

acceleration.x = (parseFloat(acceleration.x) || 0) + ' m/s\u00b2';
acceleration.y = (parseFloat(acceleration.y) || 0) + ' m/s\u00b2';
acceleration.z = (parseFloat(acceleration.z) || 0) + ' m/s\u00b2';

But, I'm really curious, when and why I can get a NaN after doing just a multiply of some values?

BTW: I've read this at MDN, as good as many related answers here at Stack Overflow, but to brought me no help or answer to my question.

回答1:

You'll get a NaN when multiplying a number with something that can't be converted to a number.

1 * '100'         // 100 because '100' will be converted to 100
1 * ''            // 0 because '' will be converted to 0
1 * 'ten'         // NaN because 'ten' can't be converted to a number
1 * []            // 0 because [] converted to number will be 0
1 * [123]         // 123 because [] will be 123
1 * ['ten']       // NaN
1 * {}            // NaN
1 * true          // 1 because true will be 1
1 * false         // 0
1 * null          // 0
1 * undefined     // NaN, undefined can't be converted to number
1 * function(){}  // NaN

The function 'isNaN' checks if the expression can be converted to a number o not. You culd check both numbers of the multiplication to confirm that both are numbers to avoid errors.

For the sake of completeness, to know if a variable is exactly NaN you must compare it with itself:

var a = NaN;
if (a != a) console.log('a is NaN');

It happens because NaN is defined to be unequal to everything, including itself.

回答2:

Are you sure, the values can never be undefined? The documentation doesn't say so, but I think it might be possible.

http://docs.phonegap.com/en/1.0.0/phonegap_accelerometer_accelerometer.md.html#Acceleration

回答3:

0 * Infinity ----> NaN at least in Chrome and Firefox.

Infinity * Infinity, 3.14159 * Infinity, and Infinity + Infinity are well behaved. They are all Infinity.

Infinity - Infinity is also NaN

You might think you have no Infinitys, but a 0.000001/0 will suffice to create one.

来源：https://stackoverflow.com/questions/17718394/when-and-why-number-evaluates-to-nan-after-multiplying-in-javascript

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