jquery - Correct way to launch a function from a href? -

following this stackoverflow example off launching twitter app within phonegap app..

i thought link below work not im guessing im lot launching function correctly?

<a onclick="twittercheck()">launch twitter app</a> 

js in example:

//twitter checker  // if mac//  var twittercheck = function(){  appavailability.check('twitter://', function(availability) {     // availability either true or false     if(availability) { window.open('twitter://user?screen_name=xerxesnoble', '_system', 'location=no');}     else{window.open('https://itunes.apple.com/ca/app/twitter/id333903271?mt=8', '_system', 'location=no'); }; }); };  //if android  var ua = navigator.useragent.tolowercase(); var isandroid = ua.indexof("android") > -1; //&& ua.indexof("mobile");  if(isandroid) {      twittercheck = function(){              appavailability.check('com.twitter.android', function(availability) {             // availability either true or false             if(availability) {window.open('twitter://user?screen_name=xerxesnoble', '_system', 'location=no');}             else{window.open('https://play.google.com/store/apps/details?id=com.twitter.android', '_system', 'location=no');};         });     }; }; 

please take @ article http://javascript.info/tutorial/functions-declarations-and-expressions. think confused way how function has been declared.

the way how call function tried - functionname(). if doesn't work, must have reasons.