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I was excited to see that My solution is almost exactly like the teacher's solution. However a minor difference. IsNaN(num1,num2) Is this proper?

```// declare program variables
var num1;
var num2;
var message;

alert("Let's do some math!"); // announce the program

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

if ( num2 === 0) { // check for zero
alert("The second number is 0. You can't divide by zero. Reload and try again.");
}

else if (isNaN(num1,num2)){ // Is not a number?
alert("At least one of the values you typed is not a number. Reload and try again.");
} else {
// build an HTML message
message = "<h1>Math with the numbers " + num1 + " and " + num2 + "</h1>";
message += num1 + " + " + num2 + " = " + (num1 + num2);
message += "<be>";
message += num1 + " * " + num2 + " = " + (num1 * num2);
message += "<be>";
message += num1 + " / " + num2 + " = " + (num1 / num2);
message += "<be>";
message += num1 + " - " + num2 + " = " + (num1 - num2);

// write message to web page
document.write(message);
}
``` The "isNaN" function only tests the first argument, the second one is ignored.

To test two arguments use the "or" operator to combine individual calls `isNaN(num1) || isNaN(num2)`

If you do this as a recap and are familiar with functions and loops you can also combine this:

function numOne(){ num1 = prompt("Please type a number"); num1 = parseFloat(num1) if (isNaN(num1)){ numOne() } return num1 }

function numTwo (){ do { num2 = prompt("Please type another number") num2 = parseFloat(num2)} while (num2===0 || isNaN(num2)) return num2 }

numOne() numTwo()

I ended up using "else if ( isNaN(num1 + num2) )", which does not seem to have been mentioned, but it works either way. Since doing math on NaN is only one of several reasons an operation might produce NaN, with this test you cannot be certain that either term was NaN to begin with.

I'm confused, then, Steven Parker ; when I ran the test using NaN one way or the other, it operated fine. Am I missing something? Here's an example:

```a = "42";
b = Infinity;
console.log("a is " + (isNaN(a) ? "" : "not ") + "NaN...");
console.log("and b is " + (isNaN(b) ? "" : "not ") + "NaN...");
console.log("but a+b is " + (isNaN(a+b) ? "" : "not ") + "NaN!");
```

a is not NaN..
and b is not NaN..
but a+b is NaN!  This difference is writing programs that work most of the time vs. programs that work all of the time. Steven Parker AHAH! Blind eyes regardless of surgery. Thank you!