Global variables in Javascript across multiple files

Question

A bunch of my JavaScript code is in an external file called helpers.js. Inside the HTML that calls this JavaScript code I find myself in need of knowing if a certain function from helpers.js has been called.

I have attempted to create a global variable by defining:

var myFunctionTag = true;

In global scope both in my HTML code and in helpers.js.

Heres what my html code looks like:

<html>
...
<script type='text/javascript' src='js/helpers.js'></script>    
...
<script>
  var myFunctionTag = false;
  ...
  //I try to use myFunctionTag here but it is always false, even though it has been se t to 'true' in helpers.js
</script>

Is what I am trying to do even feasible?

window.onload = function () { //Start Your Code } Would the the best solution - And this is Slowpoke speaking :) — Schoening, Oct 24 '13 at 20:14

score 145 · Accepted Answer · answered May 28 '10 at 21:57

145

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>

answered May 28 '10 at 21:57

tvanfosson

524,688
99
697
795

17

doesn't work for me because when trying to access from another js loaded in another html, it says the variable is not declared – ACV May 26 '16 at 20:54
Bear in mind if you use `defer` on the scripts it might fail due to the scripts not running in the order you might expect – perepm Feb 03 '22 at 10:03

score 18 · Answer 2 · answered May 29 '10 at 00:00

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.

score 17 · Answer 3 · answered Jan 02 '13 at 21:54

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>

HTML 1 and HTML 2 are identical (except the page title)... still, I made two files, just to have them physically separate. — Martin, Jan 02 '13 at 21:55

score 2 · Answer 4 · answered Jun 18 '18 at 10:35

Hi to pass values from one js file to another js file we can use Local storage concept

<body>
<script src="two.js"></script>
<script src="three.js"></script>
<button onclick="myFunction()">Click me</button>
<p id="demo"></p>
</body>

Two.js file

function myFunction() {
var test =localStorage.name;

 alert(test);
}

Three.js File

localStorage.name = 1;

score 2 · Answer 5 · answered Aug 16 '18 at 17:59

2

//Javascript file 1

localStorage.setItem('Data',10);

//Javascript file 2

var number=localStorage.getItem('Data');

Don't forget to link your JS files in html :)

answered Aug 16 '18 at 17:59

Dhanush

112
7

score 2 · Answer 6 · answered Jul 10 '20 at 02:07

If you're using node:

Create file to declare value, say it's called values.js:

export let someValues = {
  value1: 0
}

Then just import it as needed at the top of each file it's used in (e.g., file.js):

import { someValues } from './values'

console.log(someValues);

score 1 · Answer 7 · answered Nov 10 '14 at 12:01

I think you should be using "local storage" rather than global variables.

If you are concerned that "local storage" may not be supported in very old browsers, consider using an existing plug-in which checks the availability of "local storage" and uses other methods if it isn't available.

I used http://www.jstorage.info/ and I'm happy with it so far.

score 1 · Answer 8 · edited Feb 17 '15 at 14:40

1

You can make a json object like:

globalVariable={example_attribute:"SomeValue"};

in fileA.js

And access it from fileB.js like: globalVariable.example_attribute

edited Feb 17 '15 at 14:40

Opal

81,889
28
189
210

answered Feb 17 '15 at 14:16

partizanos

1,064
12
22

MrAni · Answer 9 · 2022-02-09T14:03:55.350

0

You can set window['yourVariableName'] = yourVariable; and it will make that variable global for all the files.

edited Feb 09 '22 at 14:03

answered Oct 01 '21 at 08:26

MrAni

23
6

that didn't work for me – J-Dizzle Nov 13 '21 at 16:08

Global variables in Javascript across multiple files

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