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JavaScript JavaScript Loops, Arrays and Objects Simplify Repetitive Tasks with Loops `do ... while` Loops

Posted by Dan Sutherland
Dan Sutherland
1,463 Points

Im confused by ! operators and booleans? just can't get my head around how they work in do while loops.

I was doing the do while loop course on Javascript and in the simple game where the user guesses the number, the project uses a ! operator to break out of the do while loop when the player guesses correctly but I cannot follow it.

The variable is set outside of the loop as

var correctGuess = false;

to start with, but then within the do while loop, if the player guesses correctly then

correctGuess = true;

I understand all of this, but then the while loop argument is then set as

while ( ! correctGuess)

in order to break out if the player has guessed correctly.

Why would the while loop not just have

while ( correctGuess !==true)

I just can't understand whats going on with the Boolean and the operator to cause the loop to finish?

3 Answers

andren
andren
28,558 Points
andren
andren
28,558 Points

To answer your question I'll first explain how the condition of a while statement really works.

When you provide a condition to a while statement all it cares about is whether that condition evaluates to true or false. If it evaluates to true it runs, if it evaluates to false it does not run. When you perform a comparison that operation actually results in a Boolean value that reflects the result. So 5 > 3 becomes true, 2 === 10 becomes false and so on. This means that the while statement does not really look at and care about comparisons, it only looks at the Boolean value it ultimately receives. This holds true for other conditional statements like if as well.

The second thing I'll explain is what the ! operator does. It's actually pretty simple, it takes whatever Boolean follows it and returns the opposite Boolean value. So !true evaluates to false, !false evaluates to true. That is all the operator does.

That means that when the program starts out (with correctGuess equal to false) the !correctGuess condition evaluates to true. Since the while loop only cares about whether it is passed true or false this is an acceptable value for it to receive and it runs. Once correctGuess is set equal to true within the loop the !correctGuess condition will now evaluate to false (since it always becomes the opposite of the Boolean it is used on) and once the while statement receives false as its condition it stops running.

The exact same events would unfold if the condition was correctGuess !== true since that in effect also makes a true value evaluate to false and vice versa, but it is far more verbose than just using !correctGuess.

George Columbu
George Columbu
2,076 Points

andren Absolutely the best answer i've ever seen, i had the exact same problem. You pointed exactly what i wasnt sure about. Thank you sir!

Dan Sutherland
Dan Sutherland
1,463 Points
Dan Sutherland
Dan Sutherland
1,463 Points

Andren thank you very much for taking the time to explain that to me, it makes a lot more sense now.

Much appreciated.

benyoungblood
benyoungblood
10,062 Points
benyoungblood
benyoungblood
10,062 Points

This is still confusing to me. I get what you are saying andren .

I'm going to type it out real quick as I process it through my own mind. Typing it out will help me. Maybe someone else as well.

The while loop is only going to run until ! correctGuess is met. Therefore, correctGuess is set to false, ! correctGuess means true, so the while statement is waiting on ! correctGuess to happen. Once it sees that correctGuess has been set to true, then ! correctGuess becomes false.

Similarly: 

var randomNumber = getRandomNumber(10);
var guess;
var guessCount = 0;

function getRandomNumber( upper ) {
  var num = Math.floor(Math.random() * upper) + 1; 
  return num;
}

do {
  guess = prompt('Guess a number between 1 and 10');
  guess = parseInt(guess);
  guessCount +=1;
}while(guess !== randomNumber)

In this case, guess !== randomNumber is true until the user guesses the random number. Once the user guesses the random number, guess !== randomNumber becomes false, because that while statement is no longer true. So the program exits on a false statement.

Okay, I think that helped me figure it out.