檢查是否加載了jquery

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

檢查是否安裝了jquery

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

測試 jquery 是否有效

window.onload = function() {
    if (window.jQuery) {  
        // jQuery is loaded  
        console.log("jQuery has loaded!");
    } else {
        // jQuery is not loaded
        console.log("jQuery has not loaded!");
    }
}

jquery檢查文檔是否加載

$( handler )
$( document ).ready( handler )
$( "document" ).ready( handler )
$( "img" ).ready( handler )
$().ready( handler )