Math.acosh()

console.log(Math.acosh(0.999999999999));
// Expected output: NaN

console.log(Math.acosh(1));
// Expected output: 0

console.log(Math.acosh(2));
// Expected output: 1.3169578969248166

console.log(Math.acosh(2.5));
// Expected output: 1.566799236972411

Math.acosh(x)

Math.acosh(-1); // NaN
Math.acosh(0); // NaN
Math.acosh(0.5); // NaN
Math.acosh(1); // 0
Math.acosh(2); // 1.3169578969248166

Math.acosh =
  Math.acosh ||
  function (x) {
    return Math.log(x + Math.sqrt(x * x - 1));
  };