从javascript代码在DOM中添加外部脚本

Question

I have included 3 external js files at the end of body. Suppose my document already contains a js named as insertlibs.js and here is the code

var script1 = document.createElement('script');
script1.src='http://code.jquery.com/jquery-latest.js';
script1.type='text/javascript';
document.getElementsByTagName('Body').item(0).appendChild(script1);

// Similar way to include underscore

var script2 = document.createElement('script');
script2.src='hhttp://documentcloud.github.com/backbone/backbone-min.js';
script2.type='text/javascript';
document.getElementsByTagName('Body').item(0).appendChild(script2);  

But what is happening sometimes, it is throwing an error that $ is not defined and I tried to debug in Firefox and there is a parallel download occurring for jquery and backbone and sometimes backbone library getting download earlier than jQuery which is causing this error.

As far as i know that if a script tag is included, it will block further request So as soon as I add jquery in dom. I am confused about the workflow here happening.

So i have found the solution, I merged both the js and making a single call which is working perfectly but that does not explain me the flow happening in above case. Please help.

Answer 1

This is because you are attempting to include backbone without ensuring that jquery has been completely loaded. To correct this, you can use the script's onload attribute to attach a callback which will be fired when jquery is loaded.

For ex:

var script1 = document.createElement('script');
script1.src='http://code.jquery.com/jquery-latest.js';
script1.type='text/javascript';

// add an onload handler
script1.onload = function() {

   // load the rest of the scripts here

   var script2 = document.createElement('script');
   script2.src='hhttp://documentcloud.github.com/backbone/backbone-min.js';
   script2.type='text/javascript';
   document.getElementsByTagName('Body').item(0).appendChild(script2);
}

document.getElementsByTagName('Body').item(0).appendChild(script1);

Answer 2

As far as i know that if a script tag is included, it will block further request

No, the blocking / synchronous download is only when the tags are right in the parsed HTML (or are inserted via document.write during the parse); dynamically DOM-appended scripts load asynchronously and in parallel.

To do that but ensure that scripts are executed when their dependencies are met, you need to use AMD loaders.