I don't know why my solution works. Please check my solute:

// declare program variables
let num1;
let num2;
let message;
// announce the program
alert("Let's do some math!");

// collect numeric input
num1 = prompt("Please type a number");
num1 = parseFloat(num1);
num2 = prompt("Please type another number");
num2 = parseFloat(num2);

// check for zero
if ( num2 == 0 ){
    alert("The second number is 0. Try again");
} 

// not a number?
function parseFloat(x){
  if (isNaN(x)){
    alert("At least one of the values you typed is not a number. Reload and try again.");
  }
  return x;
}



// build an HTML message
message = "<h1>Math with the numbers " + num1 + " and " + num2 + "</h1>";
message += num1 + " + " + num2 + " = " + (num1 + num2);
message += "<br>";
message += num1 + " * " + num2 + " = " + (num1 * num2);
message += "<br>";
message += num1 + " / " + num2 + " = " + (num1 / num2);
message += "<br>";
message += num1 + " - " + num2 + " = " + (num1 - num2);

// write message to web page
if ( num2 == 0 ){
    message = NaN;
} else if ( isNaN(num1) || isNaN(num2) ){
    message = NaN;
} else{
    document.write(message);
}