How can I declare a global variable in JavaScript?
This question is related to
javascript
global-variables
Here is a basic example of a global variable that the rest of your functions can access. Here is a live example for you: http://jsfiddle.net/fxCE9/
var myVariable = 'Hello';
alert('value: ' + myVariable);
myFunction1();
alert('value: ' + myVariable);
myFunction2();
alert('value: ' + myVariable);
function myFunction1() {
myVariable = 'Hello 1';
}
function myFunction2() {
myVariable = 'Hello 2';
}
If you are doing this within a jQuery ready() function then make sure your variable is inside the ready() function along with your other functions.
Note: The question is about JavaScript, and this answer is about jQuery, which is wrong. This is an old answer, from times when jQuery was widespread.
Instead, I recommend understanding scopes and closures in JavaScript.
With jQuery you can just do this, no matter where the declaration is:
$my_global_var = 'my value';
And will be available everywhere.
I use it for making quick image galleries, when images are spread in different places, like so:
$gallery = $('img');
$current = 0;
$gallery.each(function(i,v){
// preload images
(new Image()).src = v;
});
$('div').eq(0).append('<a style="display:inline-block" class="prev">prev</a> <div id="gallery"></div> <a style="display:inline-block" class="next">next</a>');
$('.next').click(function(){
$current = ( $current == $gallery.length - 1 ) ? 0 : $current + 1;
$('#gallery').hide().html($gallery[$current]).fadeIn();
});
$('.prev').click(function(){
$current = ( $current == 0 ) ? $gallery.length - 1 : $current - 1;
$('#gallery').hide().html($gallery[$current]).fadeIn();
});
Tip: run this whole code in the console in this page ;-)
If this is the only application where you're going to use this variable, Felix's approach is excellent. However, if you're writing a jQuery plugin, consider "namespacing" (details on the quotes later...) variables and functions needed under the jQuery object. For example, I'm currently working on a jQuery popup menu that I've called miniMenu. Thus, I've defined a "namespace" miniMenu under jQuery, and I place everything there.
The reason I use quotes when I talk about JavaScript namespaces is that they aren't really namespaces in the normal sense. Instead, I just use a JavaScript object and place all my functions and variables as properties of this object.
Also, for convenience, I usually sub-space the plugin namespace with an i namespace for stuff that should only be used internally within the plugin, so as to hide it from users of the plugin.
This is how it works:
// An object to define utility functions and global variables on:
$.miniMenu = new Object();
// An object to define internal stuff for the plugin:
$.miniMenu.i = new Object();
Now I can just do $.miniMenu.i.globalVar = 3 or $.miniMenu.i.parseSomeStuff = function(...) {...} whenever I need to save something globally, and I still keep it out of the global namespace.
The best way is to use closures, because the window object gets very, very cluttered with properties.
HTML
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript" src="init.js"></script>
<script type="text/javascript">
MYLIBRARY.init(["firstValue", 2, "thirdValue"]);
</script>
<script src="script.js"></script>
</head>
<body>
<h1>Hello !</h1>
</body>
</html>
init.js (based on this answer)
var MYLIBRARY = MYLIBRARY || (function(){
var _args = {}; // Private
return {
init : function(Args) {
_args = Args;
// Some other initialising
},
helloWorld : function(i) {
return _args[i];
}
};
}());
script.js
// Here you can use the values defined in the HTML content as if it were a global variable
var a = "Hello World " + MYLIBRARY.helloWorld(2);
alert(a);
Here's the plnkr. Hope it help !
Declare the variable outside of functions
function dosomething(){
var i = 0; // Can only be used inside function
}
var i = '';
function dosomething(){
i = 0; // Can be used inside and outside the function
}
Source: Stackoverflow.com