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Showing content from https://webplatform.github.io/docs/javascript/isNaN below:

isNaN · WebPlatform Docs

isNaN Summary

Determines whether a supplied number is NaN (not a number).

Syntax

isNaN(numValue)

numValue: Required. the value to be tested against NaN

Return Value

true if the value, converted to the Number type, is the NaN; otherwise false.

Examples

isNaN(100); 
isNaN("100"); 
isNaN("ABC"); 
isNaN("10C"); 
isNaN("abc123"); 
isNaN(Math.sqrt(-1));

Remarks

You typically use this method to test return values from the parseInt and parseFloat functions.

Alternatively, a variable that contains NaN or another value could be compared to itself. If it compares as unequal, it is NaN. This is because NaN is the only value that is not equal to itself.