While循环用于JavaScript中的错误检查

Question

I am trying to get this to prompt a user to pick a color, either red, green or blue. and based off of their answer the program will display a message. I am trying to use the while loop to check to see if the user picked either red, green or blue, if they did not it will prompt again to chose only from red, green or blue.

My problem is that the program seems to be getting stuck in the while loop, no matter what they answer. I don't know what I did wrong.

I know there is probably another way to do this but this is a school assignment and I have to use a while loop for this.

var color = prompt("What is your favorite color: red, green, or blue?");
    color =color.toLowerCase();
    color = color.trim();


while(color != 'red' || color != 'blue' || color != 'green'){

     var color = prompt("Please choose from either: red, green, or blue?");
}

if (color == 'red'){

document.write('Spicy!');

}else if(color == 'green'){

    document.write('Green is such a mellow color.');

}else{

document.write('Wow! Blue is my favorite color too!');

}

Answer 1

Let's step through your while exit condition:

• colour != 'red Okay, so if it's not red, keep looping. What if it is red?
• || colour != 'blue' || colour != 'blue' Huh. But the only time we get here is if it's red. If it's red, it's certainly not blue, so the loop never ends.

Try && instead of || .

Answer 2

You define two vars called color:

change

var color = prompt("Please choose from either: red, green, or blue?");

to

color = prompt("Please choose from either: red, green, or blue?");

so it uses the colour var 'outside' the while. Also the ||'s should be &&'s.

Just to be clear:

var color;

do
{
    color=prompt("What is your favorite color: red, green, or blue?");
    color=color.toLowerCase().trim();
}
while(color!='red' && color!='blue' && color!='green');

switch(color)
{
   case 'red':
       document.write('Spicy!');
       break;

   case 'green':
       document.write('Green is such a mellow color.');
       break;

   case 'blue':
       document.write('Wow! Blue is my favorite color too!');
       break;
}

Answer 3

you define var color twice, is there any reason for that? either way this while will do the job::

    while (color) {
        if (color == "red") {
            console.log("red");
            break;
        }else if (color=="blue") {
            console.log("blue");
            break;
        }
        break;
    }

if you have color != 'red' and if this color never is pass the loop will be infinity, so in this way we break the while either way since you only expect some predefined values