How to tell whether value is NaN without using isNaN, which has false positives? [duplicate]

Question

This question already has an answer here:

• How to test whether something is identically NaN? 3 answers

How can I check whether the input value is NaN or not without using the isNaN function?

Answer 1

If you can use ECMAScript 6, you have Object.is:

return Object.is(obj, NaN);

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Otherwise, here is one option, from the source code of underscore.js:

// Is the given value `NaN`?
_.isNaN = function(obj) {
  // `NaN` is the only value for which `===` is not reflexive.
  return obj !== obj;
};

Also their note for that function:

Note: this is not the same as the native isNaN function, which will also return true if the variable is undefined.

Answer 2

Convert the input to a number, and check whether the substraction is not zero:

var x = 'value';
var is_NaN = +x - x !== 0; // The + is actually not needed, but added to show
                           // that a number conversion is made.