In spite of all the answers you already received, it is worth noting that you do not need to write a plugin to use jQuery in a function. Certainly if it's a simple, one-time function, I believe writing a plugin is overkill. It could be done much more easily by just passing the selector to the function as a parameter. Your code would look something like this:

function myFunction($param) {
   $param.hide();  // or whatever you want to do
   ...
}

myFunction($('#my_div'));

Note that the $ in the variable name $param is not required. It is just a habit of mine to make it easy to remember that that variable contains a jQuery selector. You could just use param as well.

While there is a plethora of documentation / tutorials out there, the simple answer for your question is this:

// to create a jQuery function, you basically just extend the jQuery prototype
// (using the fn alias)

$.fn.myfunction = function () {
    // blah
};

Inside that function, the this variable corresponds to the jQuery wrapped set you called your function on. So something like:

$.fn.myfunction = function () {
    console.log(this.length);
};

$('.foo').myfunction();

... will flush to the console the number of elements with the class foo.

Of course, there is a bit more to semantics than that (as well as best practices, and all that jazz), so make sure you read up on it.

To make a function available on jQuery objects you add it to the jQuery prototype (fn is a shortcut for prototype in jQuery) like this:

jQuery.fn.myFunction = function() {
    // Usually iterate over the items and return for chainability
    // 'this' is the elements returns by the selector
    return this.each(function() { 
         // do something to each item matching the selector
    }
}

This is usually called a jQuery plugin.

Example -

Yup — what you’re describing is a jQuery plugin.

To write a jQuery plugin, you create a function in JavaScript, and assign it to a property on the object jQuery.fn.

E.g.

jQuery.fn.myfunction = function(param) {
    // Some code
}

Within your plugin function, the this keyword is set to the jQuery object on which your plugin was invoked. So, when you do:

$('#my_div').myfunction()

Then this inside myfunction will be set to the jQuery object returned by $('#my_div').

See for the full story.

$(function () {
    //declare function 
    $.fn.myfunction = function () {
        return true;
    };
});

$(document).ready(function () {
    //call function
    $("#my_div").myfunction();
});

You can also use extend (the way you create jQuery plugins):

$.fn.extend(
{
    myfunction: function () 
    {
    },

    myfunction2: function () 
    {
    }
});

Usage:

$('#my_div').myfunction();

You can write your own jQuery plugins(function which can be called on selected elements) like below:

(function( $ ){
    $.fn.myFunc = function(param1, param2){
        //this - jquery object holds your selected elements
    }
})( jQuery );

Call it later like:

$('div').myFunc(1, null);

Yes, methods you apply to elements selected using jquery, are called jquery plugins and there is a good amount of info on authoring within the jquery docs.

Its worth noting that jquery is just javascript, so there is nothing special about a "jquery method".

Create a "colorize" method:

$.fn.colorize = function custom_colorize(some_color) {
    this.css('color', some_color);
    return this;
}

Use it:

$('#my_div').colorize('green');

This simple-ish example combines the best of How to Create a Basic Plugin in the jQuery docs, and answers from @Candide, @Michael.

• A named function expression may improve stack traces, etc.
• A custom method that returns this may be chained. (Thanks @Potheek.)

You can always do this:

jQuery.fn.extend({
   myfunction: function(param){
       // code here
   },
});
OR
jQuery.extend({
   myfunction: function(param){
       // code here
   },
});
$(element).myfunction(param);

It sounds like you want to extend the jQuery object via it's prototype (aka write a jQuery plugin). This would mean that every new object created through calling the jQuery function ($(selector/DOM element)) would have this method.

Here is a very simple example:

$.fn.myFunction = function () {
    alert('it works');
};

Demo

Simplest example to making any function in jQuery is

jQuery.fn.extend({
    exists: function() { return this.length }
});

if($(selector).exists()){/*do something here*/}

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Define a function in jQuery</title>
<script src=""></script>
<script>
$(document).ready(function() {
    $.fn.myFunction = function() { 
        alert('You have successfully defined your function!'); 
    }
    $(".call-btn").click(function(){
        $.fn.myFunction();
    });
});
</script> 
</head>
<body>
    <button type="button" class="call-btn">Click Here</button>
</body>
</html>