Today, we'll talk about the Javascript method Number.isNaN and the global Javascript function isNaN().

Number.isNaN() and isNaN()

Both Number.isNaN() and isNaN() return a boolean, depending on the value that has been passed.

console.log(isNaN(5)); // false
console.log(Number.isNaN(5)); //false

isNaN()

Determines whether or not a value is NaN (not a number) and returns a boolean (true or false).

console.log(isNaN(8)); // prints false, as 8 is a number
console.log(isNaN('100F')); // prints true, 100F is not a number  

isNaN() however, behaves strangely when passed non-numeric arguments at times, and it is considered unreliable compared to Number.isNaN().

This is because isNaN() tries to convert any non-numeric argument passed into it to a Number, and tests the resulting value of that instead.

// these equate to true, despite them clearly not being numbers
console.log(isNaN('NaN')); 
console.log(isNaN(undefined));
console.log(isNaN({}));
console.log(isNaN('blah')); 

Number.isNaN()

Number.isNaN() determines whether a value passed is NaN (Not a Number), using the type Number.

console.log(Number.isNaN(NaN)); // true
console.log(Number.isNaN(6));   // false

Generally, we prefer to use Number.isNaN() as a default, as it is more robust than isNaN, its original.

Below is a look at the function typeOfNaN(value), which takes in a value and uses if statements to determine which type of NaN was passed into it, we use console.log afterwards to print out which type of NaN was detected.

function typeOfNaN(value) { // tests which type of Not a Number was passed in as 'value'
  if (Number.isNaN(value)) { // if this is true
    return value + ' is not a number evaluated with Number.isNaN()';
  }
  if (isNaN(value)) { // tests if this is true
    return value + ' is not a number evaluated with isNaN()';
  }
}

console.log(typeOfNaN('100F'));
// output: "100F is not a number evaluated with isNaN()"

console.log(typeOfNaN(NaN));
// output: "NaN is not a number evaluated with Number.isNaN()"

We can also check if this value is false i.e. a Number, simply by using == false.

if(Number.isNaN(5)==false) {
  console.log('This is a number!');
}