提示用户进行特定的输入，循环并提示直到输入正确的输入

Question

I am new here and currently learning JavaScript. In the code below, how can I get it to loop and still prompt if say for example the user does not enter 1 or -1? Instead of using the else statement to output "This is not a valid input," I want it to prompt the user once again until they enter the desired input. I am not quite getting the desired out with the while loop.

var myShopping = ["Eggs", "Milk", "Potatoes", "Cereal", "Banana"];
var ord = prompt("Enter 1 for alphabetical order, " +
"and -1 for reverse order", 1);

if (ord == 1) {
    myShopping.sort();
    document.write(myShopping.join("<br />"));
} else if (ord == -1) {
    myShopping.sort();
    myShopping.reverse();
    document.write(myShopping.join("<br />"));
} else {
    document.write("That is not a valid input");
}

Answer 1

this is basically pseudo code, but should get you on the right track.. dont want to give you everything! best way to learn is through trial and error :)

var myShopping = ["Eggs", "Milk", "Potatoes", "Cereal", "Banana"];
while(true){
   //prompt here
   if(ord == 1){
       //do stuff
       break;
   }
   else if(ord == -1){
       //do stuff
       break;
   }
   else {
       //bad input
   }
}

we dont want to create the myShopping variable every time you go through the loop, so we put that outside of the loop... the prompt has to be inside incase they put in a bad character so that it will prompt them again.

some notes: document.write is pretty old and looks really bad so you should probably try to come up with a better way to do that... you may also want to put the logic of your if statements in a function you call so your code is neat

if you want to put it in a function then I would probably do something like this.

var doStuff = function(userInput){
    if(userInput == 1){
       //do stuff
       return true;
   }
   else if(userInput == -1){
       //do stuff
       return true;
   }
   else {
       //bad input
       return false;
   }
};

var myShopping = ["Eggs", "Milk", "Potatoes", "Cereal", "Banana"];
while(true){
    //prompt here
    var ord = prompt(... etc.);
    var toEndLoop = doStuff(ord);
    if(toEndLoop){
        break;
    } else {
        // they dun goofed.. tell them it was bad input and the prompt will happen again
    } 
}

Answer 2

var valid_ans = false;
while(!valid_ans) { // repeat this block until valid_ans is true
       // your code
       If (prompt===1 || prompt===-1) {
            valid_ans = true;
        }
  }

The above code should give you an idea. Keep in mind that your browser does not like multiple prompts. After two consecutive prompts the browser will ask the user if prompts should be suppressed.

Answer 3

You could use a while loop like so:

...
var promptText = "Your text";
var input = prompt(promptText);
while(input !== "1" && input !== "-1"){
  input = prompt("Your answer was not correct\n"+promptText);
}