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How to Check NaN in JavaScript

How to Check NaN in JavaScript

Ferenc Almasi • 2020 December 10 • Read time 1 min read
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Checking if a value is NaN in JavaScript can be tricky. This is because you can’t use the typeof operator since the type of NaN is surprisingly number.

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// This will return "number" as a type
typeof NaN

You also can’t do value === NaN since NaN is not equal to itself. But since it does not equal to itself, we can check the value against itself: value !== value.

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// We can't do triple equal either since NaN is not equal to itself
value === NaN // retun false
NaN === NaN   // return false

// If the value is not equal to itself, we can be sure it is NaN
value !== value

We can also use the new Number.isNaN() function introduced in ES6:

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How to check if something is NaN in JavaScript
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