Video tutorial illustrates writing custom functions in jQuery.

Following topics are covered in the video:
Function Declaration.
Function Expression.
Passing parameters.
Returning value.
Returning nothing, etc ..

HTML code
index.html

Software

Here we have 1 span tag for display purpose and a button with an id name.

jQuery code: Passing Parameter To Function
my_script.js

$(document).ready( function() {
 fun1("Oracle");
});


function fun1(text){
  $("span").append(text);
  return false;
}

function is a keyword, fun1 is a function name we have given.
Once the webpage is loaded, fun1 function will be called, and Oracle is passed to it as parameter.
Inside fun1(), the span tag is selected and the passed parameter is appended to the span tag.
Since the function doesn’t return any value back, we have written return false.

jQuery code: Passing Parameter To Function And Returning Value
my_script.js

$(document).ready( function() {

 var d = fun1(10, 20);
 $("span").append(d);

});

function fun1(a, b){
     return(a+b);
}

Once the webpage is loaded, fun1 function will be called, and 10, 20 is passed to it as parameters.
Inside fun1(), 10 and 20 are added and the resulting value is returned back to the calling function, where in it is stored in a variable called d, where we append it to the span tag.

jQuery code: Function Expression
my_script.js

$(document).ready( function() {
  $("#name").click(software);
});

var software = function() {
 $("span").append("
Microsoft ltd
");
 return false;
}

Here, once the user clicks on the button(with an id name), the custom function software gets invoked.
Since the function here doesn’t return any value, we have written return false;

jQuery code: Function Expression with Event Binding
my_script.js

$(document).ready( function() {
  $("#name").bind('click', software);
});

var software = function() {
 $("span").append("
Microsoft ltd
");
 return false;
}

Here we bind the event click to the button and then invoke the function software.

Video Tutorial: Custom Functions: jQuery

Main Purposes of writing a function:
Re-usability of code.
Reduce the number of lines of code.
Cleaner looking code.
We could easily trace, modify and maintain the code.

Helps in Object Oriented Programming, as the data is bind to methods.
We could write the code in such a way that, we could pass different parameters to the same function and get desired results. Hence reducing a lot of code overhead and cost of writing extra useless codes!