Javascript - Number NaN

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Introduction

NaN, short for Not a Number, is used to indicate when an operation intended to return a number has failed.

In ECMAScript, dividing a number by 0 returns NaN, which allows other processing to continue.

Any operation involving NaN always returns NaN.

NaN is not equal to any value, including NaN. For example, the following returns false: 

console.log(NaN == NaN);    //false

ECMAScript isNaN() function accepts a single argument, which can be of any data type, to determine if the value is "not a number."

When a value is passed into isNaN(), the value is converted into a number.

Some non-number values can be converted numbers, such as the string "10" or a Boolean value.

Any value that cannot be converted into a number causes the function to return true. Consider the following: 

console.log(isNaN(NaN));       //true 
console.log(isNaN(10));        //false - 10 is a number 
console.log(isNaN("10"));      //false - can be converted to number 10 
console.log(isNaN("blue"));    //true - cannot be converted to a number 
console.log(isNaN(true));      //false - can be converted to number 1

This example tests five different values.

isNaN() can be used on objects. The object's valueOf() method is first called to determine if the returned value can be converted into a number.

If not, the toString() method is called and its returned value is tested as well.