Does JavaScript hav If Function Exists method?

Hi,

I have several pages that load the same javascript function on the same page. I'm looking for a way to load them without have to rename the function.

I know in php you can determine if a function already exists. Does javascript have a similar method to see if the function has already been loaded?

 

er, yes. 'typeof':

<script type="text/javascript">
alert(typeof(mouseup)); // undefined
var mouseup = function(e) {
    document.onmousemove=null;
}

alert(typeof(mouseup)); // alerts 'function' - you can compare against this etc.
</script>

PHP:

one thing to keep in mind. in javascript, you can overwrite ANY name space, including the ECMA built-in functions - for example, consider this:

<script type="text/javascript">
alert(typeof(alert));
var alert = function(e) {
    document.write(e + "<br />");
}

alert(typeof(alert));

var alert = "hello!";

document.write(typeof(alert));
</script>

PHP:

it will output:
function
function
string

but the way alerts happen from the 2-nd one down is via the new document.write.

there is no danger in redefining the functions - except when this may leave circular references to dom objects - you may get memory leaks - but you will have gotten them anyway under such conditions so...

 

Thanks for the fast reply.

 

ya thnkss...i needed it too..

 

If you use an object as a container for your functions, you can just check for the existance of the object.

var toolbox = new Object();
toolbox.tool_a = function(a, b)
{
   alert(a + b);
}

Code (markup):

if( ! toolbox)
{
   load('toolbox.js');
}

Code (markup):

 

heh that kind of namespacing is cool - but what if toolbox has more methods and you need to check for a specific one only, i.e. :

var toolbox = {
    tool_a = function(a, b) {
        alert(a + b);
    },
    tool_b = function(a, b) {
        alert(a - b);
    }
};

// then this...
if(!toolbox) {
    // do something. 
    
} // ... is not specific to a tool_a or tool_b and would evaluate as false only if no functions are defined and true if either.


// you'd have to make a new object for each function...
toolbox.tool_b = function() {
    alert("hi!");
};

PHP:

 

Then you don't sub-namespace it, you declare it in the global/window namespace and check for it there.

if( ! window.my_func)
{

}

Code (markup):

 

Joe, That is an interesting way of doing that.