NaN — Testing against NaN
To tell if a value is NaN, use {{jsxref("Number.isNaN()")}} or {{jsxref("isNaN()")}} to most clearly determine whether a value is NaN — or, since NaN is the only value that compares unequal to itself, you can perform a self-comparison like x !== x.
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To tell if a value is NaN, use {{jsxref("Number.isNaN()")}} or {{jsxref("isNaN()")}} to most clearly determine whether a value is NaN — or, since NaN is the only value that compares unequal to itself, you can perform a self-comparison like x !== x.
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
For the same reason, using a BigInt value will throw an error with isNaN() and not with Number.isNaN()
Additionally, some array methods cannot find NaN, while others can. Namely, the index-finding ones ({{jsxref("Array/indexOf", "indexOf()")}}, {{jsxref("Array/lastIndexOf", "lastIndexOf()")}}) cannot find NaN, while the value-finding ones ({{jsxref("Array/includes", "includes()")}}) can
Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV isolated this documentation section, normalized formatting, removed long code blocks, and shortened it for retrieval. Verify version-sensitive details at the source.
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MDN Web Docs — files/en-us/web/javascript/reference/global_objects/nan/index.md :: Testing against NaN ↗Revision d14bee540b53 · CC-BY-SA-2.5