Global variables in Javascript across multiple files

Solution 1:

You need to declare the variable before you include the helpers.js file. Simply create a script tag above the include for helpers.js and define it there.

<script type='text/javascript' > 
  var myFunctionTag = false; 
</script>
<script type='text/javascript' src='js/helpers.js'></script>     
... 
<script type='text/javascript' > 
  // rest of your code, which may depend on helpers.js
</script>

Solution 2:

The variable can be declared in the .js file and simply referenced in the HTML file. My version of helpers.js:

var myFunctionWasCalled = false;

function doFoo()
{
    if (!myFunctionWasCalled) {
        alert("doFoo called for the very first time!");
        myFunctionWasCalled = true;
    }
    else {
        alert("doFoo called again");
    }
}

And a page to test it:

<html>
<head>
<title>Test Page</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<script type="text/javascript" src="helpers.js"></script>
</head>

<body>


<p>myFunctionWasCalled is
<script type="text/javascript">document.write(myFunctionWasCalled);</script>
</p>

<script type="text/javascript">doFoo();</script>

<p>Some stuff in between</p>

<script type="text/javascript">doFoo();</script>

<p>myFunctionWasCalled is
<script type="text/javascript">document.write(myFunctionWasCalled);</script>
</p>

</body>
</html>

You'll see the test alert() will display two different things, and the value written to the page will be different the second time.

Solution 3:

OK, guys, here's my little test too. I had a similar problem, so I decided to test out 3 situations:

One HTML file, one external JS file... does it work at all - can functions communicate via a global var?
Two HTML files, one external JS file, one browser, two tabs: will they interfere via the global var?
One HTML file, open by 2 browsers, will it work and will they interfere?

All the results were as expected.

It works. Functions f1() and f2() communicate via global var (var is in the external JS file, not in HTML file).
They do not interfere. Apparently distinct copies of JS file have been made for each browser tab, each HTML page.
All works independently, as expected.

Instead of browsing tutorials, I found it easier to try it out, so I did. My conclusion: whenever you include an external JS file in your HTML page, the contents of the external JS gets "copy/pasted" into your HTML page before the page is rendered. Or into your PHP page if you will. Please correct me if I'm wrong here. Thanx.

My example files follow:

EXTERNAL JS:

var global = 0;

function f1()
{
    alert('fired: f1');
    global = 1;
    alert('global changed to 1');
}

function f2()
{
    alert('fired f2');
    alert('value of global: '+global);
}

HTML 1:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<script type="text/javascript" src="external.js"></script>
<title>External JS Globals - index.php</title>
</head>
<body>
<button type="button" id="button1" onclick="f1();"> fire f1 </button>
<br />
<button type="button" id="button2" onclick="f2();"> fire f2 </button>
<br />
</body>
</html>

HTML 2

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<script type="text/javascript" src="external.js"></script>
<title>External JS Globals - index2.php</title>
</head>
<body>
<button type="button" id="button1" onclick="f1();"> fire f1 </button>
<br />
<button type="button" id="button2" onclick="f2();"> fire f2 </button>
<br />
</body>
</html>

Global variables in Javascript across multiple files

Solution 1:

Solution 2:

Solution 3:

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