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});

1if (typeof jQuery == "undefined") {
2  alert("JQuery is not installed");
3} else {
4  alert("JQuery is installed correctly!");
5}

1/* Answer to: "test if jquery works" */
2
3window.onload = function() {
4    if (window.jQuery) {  
5        // jQuery is loaded  
6        console.log("jQuery has loaded!");
7    } else {
8        // jQuery is not loaded
9        console.log("jQuery has not loaded!");
10    }
11}

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

1window.onload = function() {
2    if (window.jQuery) {  
3        // jQuery is loaded  
4        alert("Yeah!");
5    } else {
6        // jQuery is not loaded
7        alert("Doesn't Work");
8    }
9}
10

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 )