我知道此代码无法正常工作，为什么我找到的代码可以工作？ 是什么使错误的代码错了？

Question

This is from some exercise in some coding book. The goal is to use a for loop and jQuery to print your friends name on a different line; but every time I open it the page becomes unresponsive. I want to know exactly why in full detail.

This is the code that does not work:

var friendsName = prompt("enter a friends name");

for(friendsName; friendsName !== null;){
    $("body").append("<p>" + friendsName + "<p>");
} 

this is the code that does:

for(var listYourFriends;listYourFriends !== null;listYourFriends = prompt("enter a friends name"))
{

   if(listYourFriends !== undefined){$("body").append("<p>" + listYourFriends + " smells" + "</p>");
    }


    else if( listYourFriends === null){
        break;
    }
}

but now that I got the answer I have no idea why this code works but the other does not. The other one, though it did set off some alarms for me, seemed to me like it should have made sense. can someone explain the differences between the two?

Answer 1

Your code will go it to infinite loop because the condition friendsName !== null for for loop never gets false . For example when you enter some name into prompt it will not be changed again and will remain same throughout the code.

You need to update friendsName with new value in every loop.

 var friendsName = prompt("enter a friends name"); for(friendsName; friendsName !== null;){ $("body").append("<p>" + friendsName + "<p>"); friendsName = prompt("enter a friends name"); } 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> 

Why code works?

According to MDN the syntax for the for statement is

for ([initialization]; [condition]; [final-expression]) 

Here final-expression is the part which will be evaluated during each iteration. In the above code we are executing the following line during each iteration.

friendsName = prompt("enter a friends name");

So we can put that at the place of [final-expression] and that will behave the same. So that's the reason the code given in question works.