how to check if jquery exists code example

Example 1: check if jquery is installed

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

Example 2: how to check if jquery is loaded

window.onload = function() {
    if (window.jQuery) {  
        // jQuery is loaded  
        alert("Yeah!");
    } else {
        // jQuery is not loaded
        alert("Doesn't Work");
    }
}