Use Number.isNaN() to reliably check for NaN without type coercion, as it only returns true for actual NaN values; 2. Avoid the global isNaN() function because it coerces input and may return true for non-number values that convert to NaN; 3. Alternatively, use value !== value, which works because NaN is the only value not equal to itself, though it is less readable; 4. Never use == or === to check for NaN since NaN is not equal to anything, including itself; 5. Apply Number.isNaN() after parsing or mathematical operations to safely detect invalid number results.
In JavaScript, checking for NaN (Not-a-Number) can be tricky because NaN has the unusual behavior of being the only value that is not equal to itself. Here's how to properly check if a value is NaN, along with common pitfalls and best practices.
? Use Number.isNaN() (Recommended)
The most reliable and modern way is Number.isisNaN():
Number.isNaN(NaN); // true
Number.isNaN('hello'); // false
Number.isNaN(123); // false
Number.isNaN(undefined); // false? Important:
Number.isNaN()does not coerce the value. It only returnstrueif the value is actuallyNaN.
?? Avoid isNaN() (Global Function)
The older global isNaN() function can give misleading results because it coerces the value to a number first:
isNaN(NaN); // true
isNaN('hello'); // true (because Number('hello') is NaN)
isNaN(undefined); // true (because Number(undefined) is NaN)
isNaN({}); // trueSo even non-number values that fail to convert to a number return true, which is often not what you want.
? Alternative: Use value !== value
Since NaN is the only value in JavaScript that is not equal to itself, you can use:
function isNanValue(value) {
return value !== value;
}
isNanValue(NaN); // true
isNanValue(123); // falseThis works, but it's not very readable or commonly used. Still, it's a valid low-level check.
? Practical Tips
When checking for NaN, consider:
Where the value comes from?
If it's from parsing (e.g.,parseInt,parseFloat, or math operations),NaNis possible.Use
Number.isNaN()after a risky operation:
const result = parseFloat('not-a-number');
if (Number.isNaN(result)) {
console.log('Invalid number input');
}- Don’t use
==or===to check forNaN:
NaN === NaN; // false ← always fails
Summary
| Method | Coerces Input? | Reliable? | Use Case |
|---|---|---|---|
Number.isNaN() |
No | ? Yes | ? Preferred |
isNaN() |
Yes | ? No | Avoid |
value !== value |
No | ? Yes | Edge case / low-level |
So, use Number.isNaN() to safely and clearly check for NaN in JavaScript. It's predictable, explicit, and doesn't surprise you with type coercion.
Basically, if you're dealing with numbers and want to guard against invalid results, Number.isNaN() is your go-to.
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