在要插入的脚本标签中指定 onload 事件：

  function onLoad（）{
 $（f）.load（function（）{
 $（f）.fadein（1000）; 
}）; 
} 

 jq = document.createElement（'script'）; 
 jq.onload = onLoad; //<  - 魔术
 jq.src ='//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js'; 
 b.appendChild（jq）; 
 

另一种方法是，如果无法控制脚本插入代码，可以使用一个轮询器： p>

 （function（）{
 function onLoad（）{...} //要运行的代码

 if（'jQuery'in window）onLoad（）; 
 else {
 var t = setInterval（function（）{//运行poller 
 if（'jQuery'in window ）{
 onLoad（）; 
 clearInterval（t）; //停止轮询
} 
}，50）; 
} 
}）（） ; 
 

I am dynamically inserting the jQuery library on a page via <script> tag:

jq = document.createElement('script');
jq.setAttribute('src','//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js');
b.appendChild(jq);

Then I have some jQuery script that needs to run after the jQuery library has finished loading and is ready for use:

$(f).load(function() {
    $(f).fadein(1000);
});

How can I make it wait for jQuery to load?

Specify an onload event at the to-be-inserted script tag:

function onLoad() {
    $(f).load(function() {
        $(f).fadein(1000);
    });
}

jq = document.createElement('script');
jq.onload = onLoad;   // <-- The magic
jq.src = '//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js';
b.appendChild(jq);

An alternative way, if you cannot control the script insertion code, you can use a poller:

(function() {
    function onLoad() { ... } // Code to be run

    if ('jQuery' in window) onLoad();
    else {
        var t = setInterval(function() { // Run poller
            if ('jQuery' in window) {
                onLoad();
                clearInterval(t);        // Stop poller
            }
        }, 50);
    }
})();