JavaScript function doesn't work with variable declared before function

Question

I'd like to declare one or more variables before my function but when I do the function fails to work. I understand that variables declared inside a function are local variables that only work in the scope of the function. Why will my variable not work as a 'global' variable outside/before my function?

<script type="text/javascript">
   var visitors = document.getElementById('tb254597').value;
   function limitVisitors() {
       if ( visitors > 60 ) {
          alert("We can only accommodate 60 people on a group visit. You entered " + visitors + " visitors.");
       }
   }
</script>

do you have an element with that id? does it have a value? what errors do you get in console? — blurfus
– blurfus, Commented Mar 30, 2016 at 19:24
You need to put that script just before body ends, also, make sure you've an element with that id — Mr. Alien
– Mr. Alien, Commented Mar 30, 2016 at 19:24
Try to parse Int your var visitors and be sure your var is a number — Jorge Mejia
– Jorge Mejia, Commented Mar 30, 2016 at 19:28
@Junior, the error is that my alert popup is not firing when the condition is met. @ochi, I have an input element with that id and when I enter a value greater than 60, it fails to fire the alert popup. @Mr. Alien, the script is just before the ending body tab. Here's the input element: <input type="text" name="Field_Number_of_visitors" size="20" id="tb254597" onchange="limitVisitors(this);" /> — jimw
– jimw, Commented Mar 30, 2016 at 19:32

Community · Accepted Answer · 2020-06-20 09:12:55Z

4

Move the variable declaration and initalization inside the function, because you get the initial value from it at start and not the actual value.

If you need the variable not outside the function, it is better to keep it inside the function.
Change the value to a number with parseInt(), because you get a string from an input.
Call the function limitVisitors() somewhere, probably at submit.

function limitVisitors() {
    var visitors = parseInt(document.getElementById('tb254597').value, 10);
    if ( visitors > 60 ) {
        alert("We can only accommodate 60 people on a group visit. You entered " + visitors + " visitors.");
    }
}

limitVisitors();

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Mar 30, 2016 at 19:27

Nina Scholz

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4 Comments

jimw Over a year ago

Thanks, @Nina Scholz. I'm a complete novice and I appreciate your help. I'll use parseInt()—didn't even think about that. Regarding your point number 3, I am calling <input type="text" name="Field_Number_of_visitors" size="20" id="tb254597" onchange="limitVisitors(this);" />

rhino Over a year ago

In fact, the parseInt() part is not really necessary, as due to how JavaScript works, relational comparison between a string and a non-string value will convert both to numbers first. If the string happens to coerce to NaN, the operation will result in false. Anyway, a more useful approach would be to limit user input or notify the user of incorrect input.

jimw Over a year ago

@rhino, to "limit user input" would you recommend setting a max attribute on the input element and changing the type attribute to "number?"

rhino Over a year ago

@jimw Yes. As long as <input type="number"> is supported in your users' browsers (it's a relatively new addition), that's probably the best idea. If you want to support older browsers but don't really want to script the input from scratch, either simply check if the value is valid on submitting (not very user-friendly) or look for a library that enhances input fields (such that you can use a custom number input). To ensure quality user experience, determine a maximum value, as that provides the user with instant feedback. Also make sure you allow only integers that make sense in the context.

Mike Cluck · Accepted Answer · 2016-03-30 19:26:21Z

0

You're grabbing the value of tb254597 as soon as the page loads. This is going to be an empty string (''). You need to grab that value every time you want to check it.

Think of it as you write down a friends address. You've got their address at the time that you wrote it down. But what happens if they move? You're not going to find your friends if you go to the old address. Always check that you've got your friends latest address.

answered Mar 30, 2016 at 19:26

Mike Cluck

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Comments

Valter Júnior · Accepted Answer · 2016-03-30 19:37:46Z

0

I think you should do this:

       function limitVisitors() {
           var visitors = parseInt(document.getElementById('tb254597').value);
           if ( visitors > 60 ) {
              alert("We can only accommodate 60 people on a group visit. You entered " + visitors + " visitors.");
           }
       }

Every time you call limitVisitors you will get the value and compare.

answered Mar 30, 2016 at 19:37

Valter Júnior

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Comments

Ryan Ginnow · Accepted Answer · 2016-03-30 19:38:21Z

0

It depends on how you're calling the function. Either way, you have to pass the variable into the function in order for the function to know your value.

Somewhat similar to this:

<script type="text/javascript">
  // this parameter can be named whatever
  function limitVisitors(param1) {
    if ( param1 > 60 ) {
      alert("We can only accommodate 60 people on a group visit. You entered " + param1 + " visitors.");
    }
  }

  // Just make sure that this value gets passed to the function like this
  var visitors = document.getElementById('tb254597').value;
  limitVisitors(visitors);
</script>

And you may want to consider jQuery so that you can use the $(document).ready() function in order to run when the DOM actually loads. If you're going to allow for user input, you may want to have a event binding like a button click.

EDIT: Since you're using onchange in your HTML element, this should be all you need. Note the change in the function call:

<input type="text" name="Field_Number_of_visitors" size="20" id="tb254597" onchange="limitVisitors(this.value);" />

<script type="text/javascript">
  // this parameter can be named whatever
  function limitVisitors(param1) {
    if ( param1 > 60 ) {
      alert("We can only accommodate 60 people on a group visit. You entered " + param1 + " visitors.");
    }
  }
</script>

edited Mar 30, 2016 at 19:38

answered Mar 30, 2016 at 19:30

Ryan Ginnow

8310 bronze badges

2 Comments

jimw Over a year ago

Thanks, @Minnow990. Not sure why the parameter in the function call needed to add .value.

Ryan Ginnow Over a year ago

@jimw Normally the call to this will only select the element itself, not any of its attributes. Since you only want to call its value to the parameter in the function, you run it with this.value. Like I said, I highly recommend jQuery as it has very helpful shortcut functions.

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