Why is 'http:' appended to a string combination that only appears to start with '//' at the beginning of a string?

Is the code here?

varddisqus_shortname='slickcarousel';

(function(){
    vardsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true;
    dsq.src='//'+disqus_shortname+'.disqus.com/embed.js';
    ( document.getElementsByTagName('head')[0]|| document.getElementsByTagName('body')[0]).appendChild(dsq);
})();

First of all, the format of the URL is called protocol-relative URL for the format that starts with // without the protocol (http: or https:), and it is a companion of so-called "relative paths."Correct (valid) URI specified in the URI specification (per rfc2396,3986).
　The main use of is to write (technique) that both https and http connectivity protocols to web servers can prevent security errors in either case (although there is an error), for example, accessing resources in different domains is not recommended (Reference).
　In any case, if you use a URL in this format, the agent (client/browser) wants the absolute path from the relative path to actually throw the request, so in this case, the protocol is supplemented.
　You are setting the src property of HTMLScriptElement.Following the specification, HTMLScriptElement→The SCRIPT element>Attribute definitions src→Basic HTML data types>6.4URIs:

Relative URIs are resolved to full URIs using a base URI. [RFC1808], section 3, define the normal algorithm for this process.

Therefore, the value passed to the property expands to the absolute path, so reading it will contain the protocol complementary value (absolute path string).

To put it bluntly,

const str='//'+disqus_shortname+'.disqus.com/embed.js';
console.assert(str==='//slickcarousel.disqus.com/embed.js');

const str_assign=dsq.src=str;
console.assert(str_assign==='//slickcarousel.disqus.com/embed.js');

console.assert(dsq.src!=='//slickcarousel.disqus.com/embed.js');

That's right.

If you omit the http: or https: URI schemes, the browser specifications complement the scheme.
Also, omitting the scheme is recommended by Google and others.
http://qiita.com/Sugima/items/785644372397595644ba

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