问题
I have been using the following code for loading JQuery in all of my projects. I grabbed it from http://html5boilerplate.com/. There is extensive disussion of this technique here.
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="jquery-1.7.1.min.js"><\/script>');</script>
This code works great and seems pretty darn quick once I've put it up on the interwebs, but when I open my .html file locally it takes ~10 seconds per refresh. Generally I get fed up and alter the code as follows:
<!-- uncomment when going live
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>window.jQuery || document.write('<script src="jquery-1.7.1.min.js"><\/script>');</script>-->
<!-- remove following line when going live -->
<script src="jquery-1.7.1.min.js"></script>
Am I missing something obvious here? I feel like I should not be getting the super-slow loading times, but it does resolve itself when comment out those lines.
回答1:
I'm guessing that you're not serving the HTML through a web server.
The // prefix on the url indicates that it should use the same protocol as the current resource (usually either http or https)
Since you're not serving through http and instead through a file, it's trying to look for it on your local file system, eventually timing out.
The network tab on Chrome inspector shows it trying to load the following for me:
file://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js
file:///C:/Users/[My Username]/Documents/jquery-1.7.1.min.js
It'll try to load those times and the file system (or maybe the browser) will eventually timeout.
The proper way is to serve it through a web server, either IIS if you're on Windows or Apache if you're on Linux/Mac (Apache also works in Windows, but IIS has better UI tools)
回答2:
This line:
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
Attempts to find jQuery in your file system, which means it will take a while to fail. While looking at the Network tab of the developer panel in Google Chrome, it attempts to look for the file in file://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js.Then, once it has failed to find the file, it loads jQuery (and successfully finds the file) using the next line:
<script>window.jQuery || document.write('<script src="jquery-1.7.1.min.js"><\/script>');
To remedy the problem, add https: to the src of your script tag, like so:
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
来源：https://stackoverflow.com/questions/8718662/using-html5boilerplates-code-to-load-jquery-very-slow-when-run-locally