NaN

function sanitize(x) {
  if (isNaN(x)) {
    return NaN;
  }
  return x;
}

console.log(sanitize("1"));
// Expected output: "1"

console.log(sanitize("NotANumber"));
// Expected output: NaN

NaN === NaN; // false
Number.NaN === NaN; // false
isNaN(NaN); // true
isNaN(Number.NaN); // true

function valueIsNaN(v) {
  return v !== v;
}
valueIsNaN(1); // false
valueIsNaN(NaN); // true
valueIsNaN(Number.NaN); // true

isNaN("hello world"); // true
Number.isNaN("hello world"); // false

isNaN(1n); // TypeError: Conversion from 'BigInt' to 'number' is not allowed.
Number.isNaN(1n); // false

const arr = [2, 4, NaN, 12];
arr.indexOf(NaN); // -1
arr.includes(NaN); // true

//       NaN   .
arr.findIndex((n) => Number.isNaN(n)); // 2

const f2b = (x) => new Uint8Array(new Float64Array([x]).buffer);
const b2f = (x) => new Float64Array(x.buffer)[0];
// NaN byte  .
const n = f2b(NaN);
//   NaN      .
n[0] = 1;
const nan2 = b2f(n);
console.log(nan2); // NaN
console.log(Object.is(nan2, NaN)); // true
console.log(f2b(NaN)); // Uint8Array(8) [0, 0, 0, 0, 0, 0, 248, 127]
console.log(f2b(nan2)); // Uint8Array(8) [1, 0, 0, 0, 0, 0, 248, 127]

NaN ** 0 === 1; // true