I know this topic is old, but I want to share my own ES6 solution for $_GET in JavaScript.
One Liner
window.$_GET = location.search.substr(1).split("&").reduce((o,i)=>(u=decodeURIComponent,[k,v]=i.split("="),o[u(k)]=v&&u(v),o),{});
Here is the MDN documentation on array.reduce(), arrow functions, the comma operator, destructuring assignment, and short-cicuit evaluation.
So, for a URL like google.com/webhp?q=foo&hl=en&source=lnt&tbs=qdr%3Aw&sa=X&ved=&biw=12 you've got an object:
$_GET = {
   q: "foo",
   hl: "en",
   source: "lnt",
   tbs: "qdr:w",
   sa: "X",
   ved: "",
   biw: "12"
}
and you can do things like $_GET.q or $_GET['biw'] to get what you need. Note that this approach replaces duplicated query parameters with the last-given value in the search string, which may be undesired/unexpected
URLSearchParams()
Now we also have URLSearchParams() in new browsers, which lets you do things like:
window.$_GET = new URLSearchParams(location.search);
var value1 = $_GET.get('param1');