NaN
The global NaN
property is a value representing Not-A-Number.
Property attributes of NaN |
|
---|---|
Writable | no |
Enumerable | no |
Configurable | no |
Description
NaN
is a property of the global object. In other words, it is a variable in global scope.
The initial value of NaN
is Not-A-Number — the same as the value of Number.NaN
. In modern browsers, NaN
is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it. It is rather rare to use NaN
in a program.
There are five different types of operations that return NaN
:
- Number cannot be parsed (e.g.
parseInt("blabla")
orNumber(undefined)
) - Math operation where the result is not a real number (e.g.
Math.sqrt(-1)
) - Operand of an argument is
NaN
(e.g.7 ** NaN
) - Indeterminate form (e.g.
0 * Infinity
, orundefined + undefined
) - Any operation that involves a string and is not an addition operation (e.g.
"foo" / 3
)
Examples
Testing against NaN
NaN
compares unequal (via ==
, !=
, ===
, and !==
) to any other value -- including to another NaN
value. Use Number.isNaN()
or isNaN()
to most clearly determine whether a value is NaN
. Or perform a self-comparison: NaN
, and only NaN
, will compare unequal to itself.
NaN === NaN; // false
Number.NaN === NaN; // false
isNaN(NaN); // true
isNaN(Number.NaN); // true
Number.isNaN(NaN); // true
function valueIsNaN(v) { return v !== v; }
valueIsNaN(1); // false
valueIsNaN(NaN); // true
valueIsNaN(Number.NaN); // true
However, do note the difference between isNaN()
and Number.isNaN()
: the former will return true
if the value is currently NaN
, or if it is going to be NaN
after it is coerced to a number, while the latter will return true
only if the value is currently NaN
:
isNaN('hello world'); // true
Number.isNaN('hello world'); // false
For the same reason, using a bigint
value will throw an error with isNaN()
and not with `Number.isNaN():
isNaN(1n); // TypeError: Conversion from 'BigInt' to 'number' is not allowed.
Number.isNaN(1n); // false
Additionally, some array methods cannot find NaN
, while others can.
let arr = [2, 4, NaN, 12];
arr.indexOf(NaN); // -1 (false)
arr.includes(NaN); // true
arr.findIndex(n => Number.isNaN(n)); // 2
Specifications
Specification |
---|
ECMAScript Language Specification # sec-value-properties-of-the-global-object-nan |
Browser compatibility
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