JavaScript numeric self equality [duplicate]

Question

I recently came across the following code:

function baseClamp(number, lower, upper) {
  if (number === number) {
    if (upper !== undefined) {
      number = number <= upper ? number : upper;
    }
    if (lower !== undefined) {
      number = number >= lower ? number : lower;
    }
  }
  return number;
}

The conditional at the beginning of the function is interesting.

  if (number === number) {

I tested the following in Chrome:

var number = undefined;
number === number
true
number = null
number === number
true
number = NaN
number === number
false

Does number === number simply check for NaN or is there more to it?

Answer 1

number === number simply checks if number is not NaN. NaN is the only value which is not equal to itself.

number === number can be replaced with !Number.isNaN(number) in modern browsers (see MDN docs).

Answer 2

NaN is the only value in ECMAscript that does not equal itself.

This function is equivalent to Math.min(Math.max(number, lower), upper) and seem to behave the same. It will skip any processing if number is NaN.

function baseClamp(number, lower, upper) {
  if (number === number) {
    if (upper !== undefined) {
      number = number <= upper ? number : upper;
    }
    if (lower !== undefined) {
      number = number >= lower ? number : lower;
    }
  }
  return number;
}

function mathClamp(number, lower, upper) {
  return Math.min(Math.max(number, lower), upper);
}

console.log([-10, 0, 10, NaN].map(it => [baseClamp(it, 0, 10), mathClamp(it, 0, 10)]));