how to check if jquery is ins code example

Example 1: check if jquery is loaded

$(document).ready(function(){
  if (jQuery) {  
    // jQuery is loaded  
    alert("Yeah!");
  } else {
    // jQuery is not loaded
    alert("Doesn't Work");
  }
});

Example 2: check if jquery is installed

if (typeof jQuery == "undefined") {
  alert("JQuery is not installed");
} else {
  alert("JQuery is installed correctly!");
}