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Please check my solution:

My code works, but please let me know if there's anything that may cause ERROR if I decided to continue working on this file:

```// Math alert:
alert("Let's do some math");

// Variables:
var num1;
var num2;
var message;

// Input:
num1 = Number(prompt("Please type the first number"));
num2 = Number(prompt("Please type the second number"));
if (isNaN(num1 && num2)){
alert("At least one of the values you typed is not a number. Reload the page and try again!");
message = false;
}

else if (num2 === 0){
alert("The second number is 0. You can't divide by zero. Reload the page and try again!");
message = false;
}

// HTML Message:
message = "<h1>Math between the numbers " + num1 + " and " + num2 + "</h1><br>";
message += num1 + " + " + num2 + " = " + (num1 + num2) + "<br>";
message += num1 + " - " + num2 + " = " + (num1 - num2) + "<br>";
message += num1 + " * " + num2 + " = " + (num1 * num2) + "<br>";
message += num1 + " / " + num2 + " = " + (num1 / num2) + "<br>";

if (num1 && num2) {
document.write(message);
}
```

Mod Edit: Fixed code formatting. See Steven Parker's comment below on code-formatting for information on how to do this.

Most of this looks like comments. Use the instructions for code formatting in the Markdown Cheatsheet pop-up below the "Add an Answer" area. â€ƒ Or watch this video on code formatting.

```// Math alert:
alert("Let's do some math");

// Variables:
var num1;
var num2;
var message;

// Input:
num1 = Number(prompt("Please type the first number"));
num2 = Number(prompt("Please type the second number"));
if (isNaN(num1 && num2)){
alert("At least one of the values you typed is not a number. Reload the page and try again!"); message = false;
}
else if (num2 === 0){
alert("The second number is 0. You can't divide by zero. Reload the page and try again!"); message = false;
}

// HTML Message:
message = "<h1>Math between the numbers " + num1 + " and " + num2 + "</h1><br>";
message += num1 + " + " + num2 + " = " + (num1 + num2) + "<br>";
message += num1 + " - " + num2 + " = " + (num1 - num2) + "<br>";
message += num1 + " * " + num2 + " = " + (num1 * num2) + "<br>";
message += num1 + " / " + num2 + " = " + (num1 / num2) + "<br>";

if (num1 && num2) {
document.write(message);
}
```

4 Answers

I don't think that service validates JavaScript (just HTML/CSS).

Here's a few issues I noticed:

• you can't combine values with logic (but you can use logic to combine comparisons)
• combining with AND (`&&`) means both, for either you'd use OR (`||`)
• an example of the previous 2 points would be "`if (isNaN(num1) || isNaN(num2)) {`"
• setting message to "false" doesn't do anything since it gets assigned with a string next
• the test for 0 is giving the message but not preventing the divide by 0 from happening
• the test of "`num1 && num2`" will return false if either number is 0
• if the numbers fail the criteria, it would be better to prompt again, or trigger a reload by code

If you want to validate both variables contain numbers in one check you can use something like:

```if(isNaN(num1 + num2)){
// trigger the alert
}
```

since adding a number to NaN results in NaN the alert will be triggered if either of the values are not numbers

Clever idea ... but your parentheses are unbalanced in this example.

Woops, well spotted, I updated the comment to avoid confusion, thanks for the feedback