Convert relative path to absolute using JavaScript

The most simple, efficient and correct way to do so it to just use URL api.

new URL("http://www.stackoverflow.com?q=hello").href;
//=> "http://www.stackoverflow.com/?q=hello"

new URL("mypath","http://www.stackoverflow.com").href;
//=> "http://www.stackoverflow.com/mypath"

new URL("../mypath","http://www.stackoverflow.com/search").href
//=> "http://www.stackoverflow.com/mypath"

new URL("../mypath", document.baseURI).href
//=> "https://stackoverflow.com/questions/mypath"

Performance wise, this solution is on par with using string manipulation and twice as fast as creating a tag.

Javascript will do it for you. There's no need to create a function.

var link = document.createElement("a");
link.href = "../../lib/slider/slider.css";
alert(link.protocol+"//"+link.host+link.pathname+link.search+link.hash);

// Output will be "http://www.yoursite.com/lib/slider/slider.css"

But if you need it as a function:

var absolutePath = function(href) {
    var link = document.createElement("a");
    link.href = href;
    return (link.protocol+"//"+link.host+link.pathname+link.search+link.hash);
}

Update: Simpler version if you need the full absolute path:

var absolutePath = function(href) {
    var link = document.createElement("a");
    link.href = href;
    return link.href;
}