此代码中的全局和局部范围有问题吗？

Question

I am testing this code to ensure that user only adds a number and when he does, I want to alert that number. If i test with only letters on first prompt and then change to a number on the second prompt, it still gives me a NaN. how to rectify this:

var mynum = prompt("Enter the number you wish");

function isitanumber(numb){
    while (isNaN(numb) == true){
        numb = prompt("Please add a valid number");
    } 
}

isitanumber(mynum);
alert("The number you have added is " + mynum);

Answer 1

Your code should work fine. You forgot to grab the value from your second prompt though. This could work:

var mynum = prompt("Enter the number you wish");

function isitanumber(numb){
    while (isNaN(numb) == true){
        numb = prompt("Please add a valid number");
    }
    //This sets the value you are displaying to the new input
    mynum = numb;
 }

isitanumber(mynum);

alert("The number you have added is " + mynum);

Here is this code running: https://jsfiddle.net/ruvn2uf0/

Answer 2

My best guess would be that your mynum variable doesn't get the updated value once user enters any text at initial prompt . So I would suggest you to return the value everytime back from the calling function and update your global variable.

 var mynum = prompt("Enter the number you wish"); function isitanumber(numb) { while (isNaN(numb)) { return parseInt(prompt("Please add a valid number")); //return } } mynum = isitanumber(mynum); //update the variable alert("The number you have added is " + mynum); 

You could also use do-while in this case as below:

 var mynum; do{ mynum = parseInt(window.prompt("Please enter a number", "")); }while(isNaN(mynum)); alert("The number you have added is " + mynum);