isFinite()

function div(x) {
  if (isFinite(1000 / x)) {
    return "Number is NOT Infinity.";
  }
  return "Number is Infinity!";
}

console.log(div(0));
// Expected output: "Number is Infinity!""

console.log(div(1));
// Expected output: "Number is NOT Infinity."

isFinite(testValue);

isFinite(Infinity); // false
isFinite(NaN); // false
isFinite(-Infinity); // false

isFinite(0); // true
isFinite(2e64); // true
isFinite(910); // true

isFinite(null); // true, would've been false with the
// more robust Number.isFinite(null)

isFinite("0"); // true, would've been false with the
// more robust Number.isFinite("0")