javascript - $(...).functionName() is not a function - Why it occurs and how to avoid this? -

i getting console error

$(...).functionname() not function

this function call

$("button").functionname(); 

function

$.fn.functionname = function() {        //do }(jquery); 

why error comes , how avoid error?

try using following syntax:

jquery.fn.extend({     functionname: function () {      } });