1. The basic JavaScript function

The simplest way to declare a function in JavaScript. In below example. we have written a simple function called abcd(x,y) which is simply taking two parameters x and y,.

function abcd(x,y) { 
return (x * y); 
} 
console.log(multiply(5,2)); //output: 10

2. JavaScript functions for get/set

If you want a private utility for getting/setting model's values then you can declare a function as a variable like this code below

var multiply = function(x,y) {

     return (x * y);
}
console.log(multiply(5,2));
//output: 10

3. Create your own jQuery function

This is the best way to declare functions in your own way. jQuery.fn is just an alias for jQuery.prototype.In this method we have to return the element so that we can combine jQuery functions together.

(function($){

// maintain a to the existing function
var oldEachFn = $.fn.each;

$.fn.each = function() {

    // original behavior - use function.apply to preserve context
    var ret = oldEachFn.apply(this, arguments);
	
	// add custom behaviour
	try {
		// change background colour
		$(this).css({'background-color':'orange'});
		
		// add a message
		var msg = 'Danger high voltage!';
		$(this).prepend(msg);
	}
	catch(e) 
	{
		console.log(e);
	}
	
    // preserve return value (probably the jQuery object...)
    return ret;
}

// run the $.fn.each function as normal
$('p').each(function(i,v)
{
    console.log(i,v);
});
//output: all paragrahs on page now appear with orange background and high voltage!

})(jQuery);

There are some more methods too to declare functions in jQuery. The above ones are easy and the best one to learn.