javascript : checking boolean values

Question

I have a boolean value set as a hidden variable in the form and I have the below javascript .

         $().ready(function() {

             var flag =  $('#popUpFlag').val();

             alert("flag = "+flag);

             if(flag){
                alert("flag is true");
             }else{
                alert("flag is false");
             }
        })

These are the outputs for the alert .

         flag =
         flag is false


         flag = false
         flag is false


         flag = true
         flag is false

My concern is obviously the third output . When the flag is true , why is it printing "flag is false" , instead of "flag is true" . I tested it in IE8 and FF 4

Suggestions are welcome.

Is this your only code? It should definitely not print `flag is false`. Are you doing anything else in between? — Felix Kling, Apr 27 '11 at 07:19
As @alex and @Shadow pointed out, `$('#popUpFlag').val()` will return a string, and not a boolean value. Now, every *non-empty* string evaluates to `true`, also `"false"`, but this does not explain your second case then. So you must be doing something else... — Felix Kling, Apr 27 '11 at 07:26

Guffa · Accepted Answer · 2011-04-27T07:36:07.403

No, you don't have a boolean value in the hidden field. The value in the field is always a string.

When you use the string value as if it was a boolean value, you get unexpected results. A condition is false if the value is false, 0, "" or null, but the string "false" is neither, so it's evaluated as true.

If you want a boolean value, you have to parse the string. An easy way is to simply check if the string has a specific value:

var flag =  $('#popUpFlag').val() === 'true';

score 6 · Answer 2 · answered Apr 27 '11 at 07:19

6

flag is a string, so have this instead:

if (flag === "true") {
  //true
}
else if (flag === "false") {
  //false
}

answered Apr 27 '11 at 07:19

Shadow The GPT Wizard

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Even if `flag` is the string `"true"`, it should alert `flag is true`. But it is worth pointing out that it is a string and not a boolean. – Felix Kling Apr 27 '11 at 07:20
2

And also `"false"` should be *truthy*. – alex Apr 27 '11 at 07:22
@alex: Good point. So the second case is correct either. The OP os probably doing *something* but vice versa or whatever. – Felix Kling Apr 27 '11 at 07:23

score 0 · Answer 3 · edited May 23 '17 at 10:27

0

Hmm... I suspect that the value you are using is a string, so you're seeing the value correctly in the alert, but not when it tries to look at it like a boolean.

How can I convert a string to boolean in JavaScript?

Just try ocnverting to boolean and see if it still gives you the same issue

edited May 23 '17 at 10:27

Community

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answered Apr 27 '11 at 07:26

Cyril Gupta

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javascript : checking boolean values

3 Answers3