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## Testing for NaN using isNaN() using javascript

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### Testing for NaN using isNaN() using javascript

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``window.isNaN()``
The global function isNaN() can be used to check if a certain value or expression evaluates to NaN. This function (in short) first checks if the value is a number, if not tries to convert it (*), and then checks if the resulting value is NaN.

For this reason, this testing method may cause confusion.
(*) The "conversion" method is not that simple, see ECMA-262 18.2.3 for a detailed explanation of the algorithm.

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``````isNaN(NaN); // true
isNaN(1); // false: 1 is a number
isNaN(-2e-4); // false: -2e-4 is a number (-0.0002) in scientific notation
isNaN(Infinity); // false: Infinity is a number
isNaN(true); // false: converted to 1, which is a number
isNaN(false); // false: converted to 0, which is a number
isNaN(null); // false: converted to 0, which is a number
isNaN(""); // false: converted to 0, which is a number
isNaN(" "); // false: converted to 0, which is a number
isNaN("45.3"); // false: string representing a number, converted to 45.3
isNaN("1.2e3"); // false: string representing a number, converted to 1.2e3
isNaN("Infinity"); // false: string representing a number, converted to Infinity
isNaN(new Date); // false: Date object, converted to milliseconds since epoch
isNaN("10\$"); // true : conversion fails, the dollar sign is not a digit
isNaN("hello"); // true : conversion fails, no digits at all
isNaN(undefined); // true : converted to NaN
isNaN(); // true : converted to NaN (implicitly undefined)
isNaN(function(){}); // true : conversion fails
isNaN({}); // true : conversion fails
isNaN([1, 2]); // true : converted to "1, 2", which can't be converted to a number
``````
This last one is a bit tricky: checking if an Array is NaN. To do this, the Number() constructor first converts the array to a string, then to a number; this is the reason why isNaN([]) and isNaN([34]) both return false, but isNaN([1, 2]) and isNaN([true]) both return true: because they get converted to "", "34", "1,2" and "true" respectively. In general, an array is considered NaN by isNaN() unless it only holds one element whose string representation can be converted to a valid number.

Number.isNaN()
In ECMAScript 6, the Number.isNaN() function has been implemented primarily to avoid the problem of
window.isNaN() of forcefully converting the parameter to a number. Number.isNaN(), indeed, doesn't try to
convert the value to a number before testing. This also means that only values of the type number, that are
also NaN, return true (which basically means only Number.isNaN(NaN)).

When the Number.isNaN is called with one argument number, the following steps are taken:
1. If Type(number) is not Number, return false.
2. If number is NaN, return true.
3. Otherwise, return false.

Some examples:

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``````// The one and only
Number.isNaN(NaN); // true
// Numbers
Number.isNaN(1); // false
Number.isNaN(-2e-4); // false
Number.isNaN(Infinity); // false
// Values not of type number
Number.isNaN(true); // false
Number.isNaN(false); // false
Number.isNaN(null); // false
Number.isNaN(""); // false
Number.isNaN(" "); // false
Number.isNaN("45.3"); // false
Number.isNaN("1.2e3"); // false
Number.isNaN("Infinity"); // false
Number.isNaN(new Date); // false
Number.isNaN("10\$"); // false
Number.isNaN("hello"); // false
Number.isNaN(undefined); // false
Number.isNaN(); // false
Number.isNaN(function(){}); // false
Number.isNaN({}); // false
Number.isNaN([]); // false
Number.isNaN([1]); // false
Number.isNaN([1, 2]); // false
Number.isNaN([true]); // false``````
NaN
NaN stands for "Not a Number." When a mathematical function or operation in JavaScript cannot return a specific number, it returns the value NaN instead.

It is a property of the global object, and a reference to Number.NaN

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``````window.hasOwnProperty('NaN'); // true
NaN; // NaN``````
Perhaps confusingly, NaN is still considered a number.

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``typeof NaN; // 'number'``
Don't check for NaN using the equality operator. See isNaN instead.

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``````NaN == NaN // false
NaN === NaN // false``````