concider the following code:
var postValue = "99"; var number = new Number(postValue); //also tried var number = parseInt(postValue); if (isNaN(number)) { alert('please enter a number'); } How come isNan(number) in both cases is false?
- 1Works fine for me in both cases (FF24).Sirko– Sirko2013-09-18 08:57:04 +00:00Commented Sep 18, 2013 at 8:57
- 1It's working as it should. isNaN(x) returns true if x is not a number else if x is a number it returns false. In your code postValue is a number as it has been implicitly converted to int from string.And number gets the value from postValue which is in your case 99. So as 99 is a number it returns false and the alert doesn't show up.Md Ashaduzzaman– Md Ashaduzzaman2013-09-18 09:12:00 +00:00Commented Sep 18, 2013 at 9:12
- The problem was elsewhere. I always use simplified versions of my problem. I used a hashtable to store errors. So if postValue is a string, the table gets filled with error info, but then after that when I validate with an integer, the validation is ok, but the value is still present in the table. stupidVincent– Vincent2013-09-18 09:17:31 +00:00Commented Sep 18, 2013 at 9:17
- So, when you validate with an integer you don't want to update the hashtable. Is that what you are talking about? It would be better if you could show something you worked on.Md Ashaduzzaman– Md Ashaduzzaman2013-09-18 12:24:20 +00:00Commented Sep 18, 2013 at 12:24
- Yes I do wan't to update in that case. Problem was: wrong input -> put error in hashtable. But when I corrected the input, I forgot the error was still in the hastable..Vincent– Vincent2013-09-19 09:24:26 +00:00Commented Sep 19, 2013 at 9:24
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3 Answers
isNAN = "is NOT a number". This means that, if isNaN(number) === false, then number IS a number:
var postValue = "99"; var number = parseInt(postValue); if (isNaN(number)) { alert('please enter a number'); } 
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parseInt() will convert your string to an integer. To do so, it reads characters from the left of the string to find as many number characters as it can, then returns that number.
In this case, since "99" is all numeric characters, the result is the number 99. Which is very clearly "not-not a number" and therefore isNaN returns false.
isNaN will return true if the input string starts with something that isn't a digit.
answered Sep 18, 2013 at 9:00
Niet the Dark Absol
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isNaN() : Is Not a Number, would return false when the argument is not a NUMBER. Which is the case for you code. You always get a number in both the cases.
