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isFinite function determines if the passed argument is finite value. It checks if its argument is not NaN , or negative infinity or positive positive infinity. isNaN on the other hand determines whether the argument passed is a NaN or not. This function is necessary because of the nature of NaN .
isFinite is a function property of the global object. You can use this function to determine whether a number is a finite number. The isFinite function examines the number in its argument. If the argument is NaN , positive infinity, or negative infinity, this method returns false ; otherwise, it returns true .
isNaN() returns true if a number is Not-a-Number. In other words: isNaN() converts the value to a number before testing it.
In JavaScript, NaN is short for "Not-a-Number". In JavaScript, NaN is a number that is not a legal number. The Number. isNaN() method returns true if the value is NaN , and the type is a Number.
The only difference is this:
!isNan(1/0) // --> true
isFinite(1/0) // --> false
isNaN checks whether the argument is a number or not. The Infinities (+/-) are also numerical, thus they pass the isNaN check, but don't pass the isFinite check.
** Note that any string which can be parsed as a number ("2", "3.14") will cause isNaN to return false.
Hope this helps.
PS: The answer given by user1170379 was very nearly perfect.
you might reason out [Why?] after reading this:
NaN doesn't check if the passed value is infinite or not - it checks if the input val evaluates into a "Type: Number" end-result. Because isNaN(string) is accepted, so the: isNaN("3.14") //false (which means true, the given token is duck converted into a type Number successfully )
You may understand that the input value may happen to be an unresolved brute number, even a math operation as simple as: (x/y); which in turn might yield a (+/-infinity) number.
Here x=1, y=0; meaning (1/0).Then isNaN(x/y) will first evaluate to isNaN(1/0); then to isNaN(infinity) //false. Since (1/0)=infinity is of type: "number" ie typeof(1/0) //"number" isNaN should and will return false.
You don't want to put "infinity" where an end result number is expected.
Probably for the same reason that I have implemented (isfinite(num) && isfinite(-num)) - I was getting errors from mysql complaining about putting "-nan" into the database even though I had a check for isfinite(field)...
A useful article on this subject is http://jacksondunstan.com/articles/983 which provides an optimization ((d*0.0)==0.0)
isNaN() returns true if the argument is not a number or if the argument is a non-numeric value such as a string or an object.Otherwise, It returns false.
Example: isNaN(0/0) =>true;
isNaN(2-1) =>false;
isFinite() returns true if the argument is a number other than NaN,Infinity or -Infinity.Otherwise, It returns false.
Example: isFinite("2000") =>false;
isFinite(200/2) =>true;`
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