Define it outside the scope of your functions.

<script>
var d;
function add(){
 /* these are scoped within the function, using var. */
 /* a,b, and c do not exist outside the "add" function. */
 var a = 1;
 var b = 2;
 var c = 3;
 /* this one already exists outside the function, */
 /* and we're not using var, so it's global */
 d = a+b+c;
}
add();
alert(d);
// this alerts "6"
</script>

Method 1: Explicitly declare it outside of any function scope:

var g; // This is global 

Method 2: Implicitly use a global variable by not using 'var' inside the function scope:

function x() { 
 var a = "bye"; // This is only defined in the scope of x 
 g = "hello"; // g was not var'ed, so it's a global 
} 
x(); // Run x 
alert(g); // alerts "hello" because g is global 
alert(a); // a is undefined 

Method 3: Define a variable in the window scope:

function x() { 
 window.g = "hello"; 
} 
x(); 
alert(g); 

ooh, good one Foti. I forgot about attaching to the window object.
You can attach a variable to the window explicitly, since the window object exists everywhere at once, regardless of scope.

Best practice is to combine Methods 1 and 2; declare outside the function, *and* use it within sans-var.
Or use Method 3 which is just as good.

Best practice is to avoid globals whenever possible. :) Instead, have only 1 global variable which contains an object, and then any additional variables that need to be shared between methods can be properties of that object.

var g = { 
 a : "bye", 
 b : "foo" 
} 
alert(g.a); 
alert(g.b); 

....have only 1 global variable which contains an object, and then any additional variables that need to be shared between methods can be properties of that object.

Great tip, very simple but effective method. Thanks :)

@stuartc1
Try googling for "javascript namespacing" for more details. :)