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

Question

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'; ac…

Accepted Answer

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.

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

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