CODEWITHSUNDEEP
javascript
Zze
Zze

Reputation: 18835

Recursive prompt returns undefined

I noticed something quite odd this morning when I was trying to implement an endless prompting system.

The following code returns 2 different outputs under 2 scenarios:

Scenario 1: Enter number on first prompt and ok

Output 1: number that was entered

Scenario 2: Cancel first prompt then enter number and ok

Output 2: undefined

I'm perplexed as to why this is happening. Firstly, how does this return undefined when I am checking for that in the if statement? Second, I was under the impression that in JavaScript undefined means a variable has been declared but has not yet been assigned and in this case I am assigning the var number. 

var number = null;
number = Prompt();
$("p").html("Number was " + number);

function Prompt()
{
  var input = prompt("Enter a number", "");
  if(input === null || input === "" || input === undefined || isNaN(input))
      Prompt();
  else
      return input;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>

<p></p>

Upvotes: 4

Views: 1314

Answers (3)

David Findlay
David Findlay

Reputation: 1356

When the first prompt is cancelled another Prompt() iteration is called, but it's return value is not captured anywhere. The first iteration then returns the original undefined return value.

Musa's answer shows one way to fix this. Another would be:

function Prompt() {
    var input = null;
    while(input === null) {
        input = prompt("Enter a number", "");
    }
    return input;
 }

Upvotes: -1

Mulan
Mulan

Reputation: 135377

window.prompt always returns a string or null so don't bother checking for undefined, or isNaN

Also, since window.prompt returns a string, you will need to use window.parseInt in order to reliably convert it to a Number.

function Prompt() {
  var n = window.parseInt(window.prompt("Enter a number"), 10)

  if (Number.isNaN(n))
    return Prompt()
  else
    return n
}

alert("Your number is: " + Prompt())

Upvotes: 3

Musa
Musa

Reputation: 97717

Only one code path returns a value, make it return from the recursive call. In JavaScript if a function ends without a return statement its return is undefined

function Prompt()
{
  var input = prompt("Enter a number", "");
  if(input === null || input === "" || input === undefined || isNaN(input))
      return Prompt();
  else
      return input;
}

Upvotes: 5

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