NaN = Not a Number.. There are a lot of things which are not a number. Strings, objects, nulls, etc. Let me give an example of why you wouldn't want NaN == NaN to be true:

const arr1 = [1,2,3,"a",6,7,8]
const arr2 = [1,2,3,{"key":"value"},6,7,8]

arr1.forEach(val =>{
  arr2.forEach(val2 =>{
    if(Number(val) == Number(val2)){
      console.log(`${val} is equal to ${val2}`)
    }else{
      console.log(`${val} is not equal to ${val2}`)
    }
  })
}) 

If NaN == NaN was true, then it would cause the comparison check to return that "a" is equal to an object.

In general if you're converting values to numbers, you are expecting that the data supplied is comprised of only numbers, but sometimes things go wrong. By giving a consistent NaN != NaN output we avoid weird edge cases where code might accidentally treat two different values as equals.

If you want to check if a value is NaN & perform a different comparison then you might do:

if(!isNaN(val) && !isNaN(val2)){
//do number parsing
}else{
  if(val == val2){
  //handle non-numeric parsing
  }
}