用 Javascript 加载 jQuery 并使用 jQuery

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HTML:

<html>
<head>


</head>
<body>


<div id='status'>jQuery is not loaded yet.</div>
<input type='button' value='Click here to load it.' onclick='load()' />


</body>
</html>

Script:

   <script>


load = function() {
load.getScript("jquery-1.7.2.js");
load.tryReady(0); // We will write this function later. It's responsible for waiting until jQuery loads before using it.
}


// dynamically load any javascript file.
load.getScript = function(filename) {
var script = document.createElement('script')
script.setAttribute("type","text/javascript")
script.setAttribute("src", filename)
if (typeof script!="undefined")
document.getElementsByTagName("head")[0].appendChild(script)
}


</script>

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最佳答案

There's a working JSFiddle with a small example here, that demonstrates exactly what you are looking for (unless I've misunderstood your request): http://jsfiddle.net/9N7Z2/188/

There are a few issues with that method of loading javascript dynamically. When it comes to the very basal frameworks, like jQuery, you actually probably want to load them statically, because otherwise, you would have to write a whole JavaScript loading framework...

You could use some of the existing JavaScript loaders, or write your own by watching for window.jQuery to get defined.

// Immediately-invoked function expression
(function() {
// Load the script
const script = document.createElement("script");
script.src = 'https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js';
script.type = 'text/javascript';
script.addEventListener('load', () => {
console.log(`jQuery ${$.fn.jquery} has been loaded successfully!`);
// use jQuery below
});
document.head.appendChild(script);
})();

Just remember that if you need to support really old browsers, like IE8, load event handlers do not execute. In that case, you would need to poll for the existance of window.jQuery using repeated window.setTimeout. There is a working JSFiddle with that method here: http://jsfiddle.net/9N7Z2/3/

There are lots of people who have already done what you need to do. Check out some of the existing JavaScript Loader frameworks, like:

~~https://developers.google.com/loader/~~ (no longer documented)
~~http://yepnopejs.com/~~ (deprecated)
http://requirejs.org/

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Encosia's website recommends:

<script type="text/javascript"
src="http://www.google.com/jsapi"></script>
<script type="text/javascript">
// You may specify partial version numbers, such as "1" or "1.3",
//  with the same result. Doing so will automatically load the
//  latest version matching that partial revision pattern
//  (e.g. 1.3 would load 1.3.2 today and 1 would load 1.7.2).
google.load("jquery", "1.7.2");


google.setOnLoadCallback(function() {
// Place init code here instead of $(document).ready()
});
</script>

But even he admits that it just doesn't compare to doing the following when it comes to optimal performance:

    <script src="//ajax.aspnetcdn.com/ajax/jQuery/jquery-1.7.2.min.js" type="text/javascript"></script>
<script type="text/javascript"> window.jQuery || document.write('<script src="js/libs/jquery-1.7.2.min.js">\x3C/script>')</script>
<script type="text/javascript" src="scripts.js"></scripts>
</body>
</html>

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The reason you are getting this error is that JavaScript is not waiting for the script to be loaded, so when you run

jQuery(document).ready(function(){...

there is not guarantee that the script is ready (and never will be).

This is not the most elegant solution but its workable. Essentially you can check every 1 second for the jQuery object ad run a function when its loaded with your code in it. I would add a timeout (say clearTimeout after its been run 20 times) as well to stop the check from occurring indefinitely.

var jQueryIsReady = function(){
//Your JQuery code here
}


var checkJquery = function(){
if(typeof jQuery === "undefined"){
return false;
}else{
clearTimeout(interval);
jQueryIsReady();
}
}
var interval = setInterval(checkJquery,1000);

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Using require.js you can do the same thing in a safer way. You can just define your dependency on jquery and then execute the code you want using the dependency when it is loaded without polluting the namespace:

I generally recommend this library for managing all dependencies on Javascript. It's simple and allows for an efficient optimization of resource loading. However there's some precautions you may need to take when using it with JQuery . My favourite way to deal with them is explained in this github repo and reflected by the following code sample:

<title>jQuery+RequireJS Sample Page</title>
<script src="scripts/require.js"></script>
<script>
require({
baseUrl: 'scripts',
paths: {
jquery: 'https://ajax.googleapis.com/ajax/libs/jquery/1.6.0/jquery.min'
},
priority: ['jquery']
}, ['main']);
</script>

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There is an other way to load jQuery dynamically (source). You could also use

document.write('<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"><\/script>');

It's considered bad practice to use document.write, but for sake of completion it's good to mention it.

See Why is document.write considered a "bad practice"? for the reasons. The pro is that document.write does block your page from loading other assests, so there is no need to create a callback function.

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You need to run your code AFTER jQuery finished loading

var script = document.createElement('script');
document.head.appendChild(script);
script.type = 'text/javascript';
script.src = "//ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js";
script.onload = function(){
// your jQuery code here
}

or if you're running it in an async function you could use await in the above code

var script = document.createElement('script');
document.head.appendChild(script);
script.type = 'text/javascript';
script.src = "//ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js";
await script.onload
// your jQuery code here

If you want to check first if jQuery already exists in the page, try this

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From the DevTools console, you can run:

document.getElementsByTagName("head")[0].innerHTML += '<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"><\/script>';

Check the available jQuery version at https://code.jquery.com/jquery/.

To check whether it's loaded, see: Checking if jquery is loaded using Javascript.

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ES6 Async version, allowing you to use it as a function anywhere you like (similar to dynamic import):

/**
* @see https://stackoverflow.com/a/10113434
*/
function loadScript(cfg) {
const {
url,
global: globalName
} = cfg;
return new Promise((r, j) => {
// Load the script
const script = document.createElement("script");
script.type = 'text/javascript';
script.onload = function () {
r(globalName && window[globalName]);
};
script.onerror = function (err) {
j(err);
};
script.src = url;
document.getElementsByTagName("head")[0].appendChild(script);
});
}

Usage example:

const jQuery = await loadScript({
url: 'https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js',
global: 'jQuery'
});


jQuery('body')

NOTE: if you get undefined, you probably did not provide the correct global name.