How to declare a global variable in JavaScript

How can I declare a global variable in JavaScript?

Solution 1:

If you have to generate global variables in production code (which should be avoided) always declare them explicitly:

window.globalVar = "This is global!";

While it is possible to define a global variable by just omitting var (assuming there is no local variable of the same name), doing so generates an implicit global, which is a bad thing to do and would generate an error in strict mode.

Solution 2:

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.

Solution 3:

_{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.

Old, bad answer

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 ;-)

Solution 4:

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.

Solution 5:

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
}