如果未加载纯Javascript和后备，则检测并附加jQuery

Question

Does anybody know a way to detect if the jQuery library was loaded and if not append it and start a script as a fallback solution after it's loaded to the DOM?

Here's my script:

if (typeof jQuery == 'undefined') {
    // if jQuery Library is not loaded
    var script = document.createElement( 'script' );
    script.type = 'text/javascript';
    script.src = 'http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js';
    document.body.appendChild(script);
    startScript();
} else {
    // cool, jQuery Library is loaded
    startScript();
}

function startScript() {
    $.noConflict();
    jQuery(document).ready(function($){
    .... // my jquery code
    });
}

Of course, above appends jQuery correctly but doesn't do anything because the script starts immidiately after appending. Any hint on this?

I appreciate any answer. Thanks!

Answer 1

You're fine except, as you said, that you can't call startScript immediately after appending the new script element. Instead, do a setTimeout loop:

// ...
document.body.appendChild(script);
maybeStart();

Where maybeStart looks like this (not within the body of the if , function declarations aren't valid there):

function maybeStart() {
    if (typeof jQuery === "undefined") {
        setTimeout(maybeStart, 100);
    }
    else {
        startScript();
    }
}

That checks if jQuery is loaded and, if not, waits a 10th of a second and checks again. When jQuery is found, it calls your startScript .

You might want to put a time limit in there (in case jQuery never loads, so you don't loop forever).

Alternately, you can hook a "load"-style event on the script element, but the mechanics of it vary from browser to browser. On some browsers it's just load as with, say, an img element; on IE (at least older versions), you have to use readystatechange , and on some versions of IE both work. So you have to keep a flag so you know whether you've fired off your script, etc., and...well, the above is less complicated in the case where you know the script creates a well-known global symbol (as is the case with jQuery).