Having confusion regarding how to pass value in a var through prompt

Question

<script>
var p=prompt("how old are you","");
if(p)
alert(p+" is your age");
else
alert("You dint entered any input or you have entered a non-integervalue");
</script>

This is my javascript code. Suppose i enter my age 0.
Then p=0 this implies the else part of the code will execute. But the code is executing the if part!
Why is it happening?
I'm new to Web-development , Please Help.
Thank You!

Possible duplicate of [In JavaScript, why is "0" equal to false, but when tested by 'if' it is not false by itself?](https://stackoverflow.com/questions/7615214/in-javascript-why-is-0-equal-to-false-but-when-tested-by-if-it-is-not-fals) — FuzzyTree, Oct 22 '17 at 05:43
Because it returns a string of zero `"0"` not a number of `0`. only empty strings `""` get coerced into `false` — Jay Harris, Oct 22 '17 at 05:44
That's because your if condition is checking for any input. It doesn't know you are expecting a number or the amount. Check this [**JsFiddle Demo**](https://jsfiddle.net/3rpmsoxh/) — NewToJS, Oct 22 '17 at 05:44

score 1 · Answer 1 · answered Oct 22 '17 at 05:45

1

Just write this code :

var p=prompt("how old are you","");
alert(typeof(p));

You will see that it is a string. "0" is a string, so it will be evaluated to true as it is not empty or null.

So, parse it to a number before doing anything.

answered Oct 22 '17 at 05:45

Miraj50

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score 1 · Accepted Answer · answered Oct 22 '17 at 05:47

parseInt() the result of the prompt, that means, parsing the string result to an integer

Now, if the input is "0", it is parsed to 0, and 0 == false.

Thus the "else" condition would take place, as required.

Also, if a non-integer string is entered, such as alphabets, the result would be a NaN and the else part would occur

var p=parseInt(prompt("how old are you",""));
if(p) {
alert(p+" is your age");
}
else {
alert("You dint entered any input or you have entered a non-integervalue");
}

score 1 · Answer 3 · answered Oct 22 '17 at 05:57

If you want to ensure the prompt input is number, you should use isNaN function to check the input.

    var p=prompt("how old are you",""); 
    if(!isNaN(p)) alert(p+" is your age"); 
    else alert("You dint entered any input or you have entered a non-integervalue");

Having confusion regarding how to pass value in a var through prompt

3 Answers3