isNaN()
The isNaN()
function determines whether a value is
NaN
or not. Because coercion inside the isNaN
function can be surprising, you may alternatively
want to use Number.isNaN()
.
Syntax
isNaN(value)
Parameters
value
-
The value to be tested.
Return value
true
if the given value is NaN
; otherwise,
false
.
Description
The convenience of an isNaN function
Unlike all other possible values in JavaScript, it is not possible to use the equality
operators (== and ===) to compare a value against NaN
to determine whether
the value is NaN
or not, because both NaN == NaN
and
NaN === NaN
evaluate to false
. The isNaN()
function provides a convenient
equality check against NaN
.
Origin of NaN values
NaN
values are generated when arithmetic operations result in
undefined or unrepresentable values. Such values do not necessarily
represent overflow conditions. A NaN
also results from attempted coercion
to numeric values of non-numeric values for which no primitive numeric value is
available.
For example, dividing zero by zero results in a NaN
— but dividing other
numbers by zero does not.
Confusing special-case behavior
Since the very earliest versions of the isNaN
function specification, its
behavior for non-numeric arguments has been confusing. When the argument to the
isNaN
function is not of type Number, the value is first coerced to a
Number. The resulting value is then tested to determine whether it is NaN
.
Thus for non-numbers that when coerced to numeric type result in a valid non-NaN numeric
value (notably the empty string and boolean primitives, which when coerced give numeric
values zero or one), the "false" returned value may be unexpected; the empty string, for
example, is surely "not a number." The confusion stems from the fact that the term, "not
a number", has a specific meaning for numbers represented as IEEE-754 floating-point
values. The function should be interpreted as answering the question, "is this value,
when coerced to a numeric value, an IEEE-754 'Not A Number' value?"
ECMAScript 2015 contains the Number.isNaN()
function.
Number.isNaN(x)
is a reliable way to test whether x
is
NaN
or not. Even with Number.isNaN
, however, the meaning of
NaN
remains the precise numeric meaning and not, "not a number".
Alternatively, in the absence of Number.isNaN
, the expression
(x != x)
is a more reliable way to test whether variable x
is
NaN
or not, as the result is not subject to the false positives that make
isNaN
unreliable.
A polyfill for isNaN
would be (the polyfill leverages the unique
never-equal-to-itself characteristic of NaN
):
const isNaN = function(value) {
const n = Number(value);
return n !== n;
};
Examples
isNaN(NaN); // true
isNaN(undefined); // true
isNaN({}); // true
isNaN(true); // false
isNaN(null); // false
isNaN(37); // false
// strings
isNaN('37'); // false: "37" is converted to the number 37 which is not NaN
isNaN('37.37'); // false: "37.37" is converted to the number 37.37 which is not NaN
isNaN("37,5"); // true
isNaN('123ABC'); // true: parseInt("123ABC") is 123 but Number("123ABC") is NaN
isNaN(''); // false: the empty string is converted to 0 which is not NaN
isNaN(' '); // false: a string with spaces is converted to 0 which is not NaN
// dates
isNaN(new Date()); // false
isNaN(new Date().toString()); // true
// This is a false positive and the reason why isNaN is not entirely reliable
isNaN('blabla'); // true: "blabla" is converted to a number.
// Parsing this as a number fails and returns NaN
Useful special-case behavior
There is a more usage oriented way to think of isNaN()
: If
isNaN(x)
returns false
, you can use x
in an
arithmetic expression not making the expression return NaN
. If it returns
true
, x
will make every arithmetic expression return
NaN
. This means that in JavaScript, isNaN(x) == true
is
equivalent to x - 0
returning NaN
(though in JavaScript
x - 0 == NaN
always returns false, so you can't test for it). Actually,
isNaN(x)
, isNaN(x - 0)
, isNaN(Number(x))
,
Number.isNaN(x - 0)
, and Number.isNaN(Number(x))
always return
the same and in JavaScript isNaN(x)
is just the shortest possible form to
express each of these terms.
You can use this, for example, to test whether an argument to a function is arithmetically processable (usable "like" a number), or if it's not and you have to provide a default value or something else. This way you can have a function that makes use of the full versatility JavaScript provides by implicitly converting values depending on context.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-isnan-number |
Browser compatibility
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