While loop exits instead of returning to the loop -Javascript

Question

Trying to get only a number answer. The inner while loops exits instead of returning back into the loop. I'm not sure why.

var numPlays = prompt("How many games do you want to play?");
if (isNaN(numPlays) == true) {
 while (isNaN(numPlays) == true) {
 numPlays = prompt("That's not a number. Please pick a number.");
 }
} else if (numPlays == 0) {
 while (numPlays == 0) {
 numPlays = prompt("You have to at least play one game. Please pick a number.");
 }
} else if (numPlays == 1) {
 alert("OK! Let's play " + numPlays + " game!");
} else {
 alert("OK! Let's play best of " + numPlays + "!");
}

• Pointy: You've got two while loops. Each one only checks for a particular kind of error. If somebody types 0, the first one is skipped. The second one will report that error and ask for another number. If you then type "XYZ", the second while loop will exit because "XYZ" is not equal to zero.
• randomgibberish: How do I return to the outer if/else statement if the while loops are false? For example, if I first typed in 0, then 2, how do I get the computer to alert "OK! Let's play best of " + numPlays + "!"
• zerkms: isNaN checks if the value is a NaN (which is a special IEEE754 value). It's not for checking if it's a number.

Answer 1

There is no need to ever enter a loop.

A quick example

// Wrap in a function so we can start over
function start() {
 var numPlays = prompt("How many games do you want to play?");
 // edited this to check for 0
 if (isNaN(numPlays) || numPlays == 0) {
 alert('Nope, bad answer, try again!');
 return start();
 } else if (numPlays == 1) {
 alert('Ok, lets play 1 game!');
 } else {
 alert('Ok, lets play the best of ' + numPlays + 'games!');
 // A little aside
 // var neededToWin = (Math.floor((numPlays / 2)) + 1);
 // alert('Ok, you need to win ' + neededToWin + ' out of ' + numPlays + ' games to beat me!');
 }
}

Technically you would want the best out of 'half of the games + 1'. So to win, you need to win 7/12 games. You could just find this out by dividing

• Pointy: Your function should probably return the value when it's satisfied that it's OK.
• Dustin Stiles: up to the OP to choose what to do next.

Answer 2

Because of the if/elseif structure; only one of them can match. The first time it runs through, it asks for the number at line 1, then decides which branch of the if/elseif/else structure to go through. It doesn't matter that you're changing the value of numPlays later, the decision was already made!

You probably want a recursive call in here.

function getNumPlays(message) {
 if (!message) { message = "How many games do you want to play?"; }
 var numPlays = parseInt(prompt(message), 10);
 if (isNaN(numPlays)) {
 return getNumPlays("That's not a number.");
 } else if (numPlays < 1) {
 return getNumPlays("You have to at least play one game.");
 }
 return numPlays;
}
var numChosen = getNumPlays();
if (numPlays === 1) {
 alert("OK! Lets play " + numChosen + " game!");
} else {
 alert("OK! Let's play best of " + numChosen + "!");
}

PS: note the parseInt(..., 10) in there. This converts to either a number, or NaN. The '10' part is important; it specifies what base the number is in. Octal numbers are written "011", and sometimes the javascript engine won't know if you want octal or decimal.

Answer 3

An empty input would evaluate to false ('ok' in your while loop).

Try using parseInt(), which returns NaN for empty strings and non-numbers, Test a few of your own examples using this:

isNaN(parseInt(prompt("enter number")))

Answer 4

var numPlays = prompt("How many games do you want to play?");
while (isNaN(numPlays) == true || numPlays == 0) {
 if(isNaN(numPlays) == true)
 numPlays = prompt("That's not a number. Please pick a number.");
 else 
 numPlays = prompt("You have to at least play one game. Please pick a number.");
 }
if (numPlays == 1) 
 alert("OK! Let's play " + numPlays + " game!");
else 
 alert("OK! Let's play best of " + numPlays + "!");

Do we have to have all those nested loops... is that a requirement? can we make it simpler?