jquery check if document loaded

1$(document).ready(function(){
2  if (jQuery) {  
3    // jQuery is loaded  
4    alert("Yeah!");
5  } else {
6    // jQuery is not loaded
7    alert("Doesn't Work");
8  }
9});

1jQuery offers several ways to attach a function that will run when the DOM is ready. All of the following syntaxes are equivalent:
2
3$( handler )
4$( document ).ready( handler )
5$( "document" ).ready( handler )
6$( "img" ).ready( handler )
7$().ready( handler )