Comparison of JavaScript Boolean variables in IF statements

Question

In my Spring MVC project I am getting the one Boolean variable as a model attribute from the controller in my JSP view. based on the Boolean variable's value I have to execute the Js code. So I am comparing the value in the if statement. but it is not giving me the correct output:

The code is something like this:

In spring controller the Boolean variable is:

boolean isAdmin;

which is recieved by the JSP view and I can access it as follows:

$(isAdmin)

In my JSP view I am doing as follows:

<script>
  alert('${isAdmin}')  // which gives the correct value .i.e : true / false

  if('${isAdmin}'){

   // some code
  }

</script>

So in the above if statement if the value of the isAdmin is false than it should not be execute if block but it does executed even the value is true.

I also compared the value of the Boolean variable as follow:

if('${isAdmin}' == true )
if('${isAdmin}' === true)

But these both are also not working.

So correct me where I am doing the wrong? and what is the correct comparison method for this?

The answer is in your question. You're wrapping it in single quotes, which means it's not true but 'true', which is text. — user5734311
– user5734311, Commented Aug 24, 2017 at 7:14
@ChrisG but without wrapping it in single quote gives compile time error — Akshay Pethani
– Akshay Pethani, Commented Aug 24, 2017 at 7:17
@ChrisG Yes, the reason for the single quote is that EL expressions should be wrapped by quotes. — Suresh Atta
– Suresh Atta, Commented Aug 24, 2017 at 7:18

Pablo Lozano · Accepted Answer · 2017-08-24 08:04:27Z

3

As far as I understand your scenario, you have a boolean variable isAdmin in the server side (JSP). You want to inject it in a javascript code, but it is not working, because you are creating a string literal instead of a boolean one:

if ('true') {
  console.log('This always executes');
}

if('false') {
  console.log('This always executes, too');
}

This happens because any non empty string is considered a "truthy" value by Javascript.Remove the quotes and it will work. But for the sake of cleanliness, I would move the Javascript code to its own file and I will create a hidden field in the HTML code to store the isAdmin value:

<input id="isAdminFlag" type="hidden" value="${isAdmin}" />


if ($('isAdminFlag').val() == 'true') { ... }

edited Aug 24, 2017 at 8:04

answered Aug 24, 2017 at 7:21

Pablo Lozano

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2 Comments

Suresh Atta Over a year ago

Anyway in the end you checking against 'true'. Why not directly doing it and having extra input field ?

Vazgen Torosyan Over a year ago

It's one of way injecting server side generated value in JS code, Second way is using <![CDATA[ ]]>. It works with Theamleaf, I didn't tested it in JSP

Suresh Atta · Accepted Answer · 2017-08-24 07:17:44Z

2

Since the resulted value is a String and you are expecting a boolean, '${isAdmin}' should be check against a boolean String value. As already someone commented, it should be '${isAdmin}' === 'true'.

if('${isAdmin}' === 'true') {
  // TODO
}

The reason why it is always executing is Javascript truthify the if expression. For example if("false") is evalutes to true and executes

if("false"){
alert("What ??");
}

answered Aug 24, 2017 at 7:17

Suresh Atta

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Comments

Nina Scholz · Accepted Answer · 2017-08-24 07:10:21Z

1

Why not just use the value without quotes?

if (${isAdmin}) {

answered Aug 24, 2017 at 7:10

Nina Scholz

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3 Comments

Akshay Pethani Over a year ago

Yes it gives the compile time error in <script> tag

Nina Scholz Over a year ago

@AkshayPethani, could you use <%= ${isAdmin}) %>?

Akshay Pethani Over a year ago

@NinaScholz I tried this too. But it also not worked.

Vazgen Torosyan · Accepted Answer · 2017-08-24 07:43:37Z

0

Your rendered js code is wrong. Rendering engine should put the value of 'isAdmin' in js code, but actually not doing. You need to add your script like this

 <script>
<![CDATA[
     var isTrueSet = ('${isAdmin}'=== 'true');

      if(isTrueSet ){
          // some code
  }
]]>
</script>

edited Aug 24, 2017 at 7:43

answered Aug 24, 2017 at 7:35

Vazgen Torosyan

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