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How do I force browser not to convert 'comma' to 2C symbols using JS?

I'm changing current user's path through a function:
function setSomeValue(someValues) {
var query = '';
for (var i = 0; i < someValues.length; i++) {
query += someValues[i] + ',';
}
if ('URLSearchParams' in window) {
var searchParams = new URLSearchParams(window.location.search);
searchParams.set("paramName", query);
var newRelativePathQuery = window.location.pathname + '?' + searchParams.toString();
history.pushState(null, '', newRelativePathQuery);
}
}
As you can see, I'm adding to user's location new words and want new location to be like this:
www.site.com?paramName=value1,value2,
But browser converts my commas into %2C so I get this:
www.site.com?paramName=value1%2Cvalue2%2C
What should be done to make pushing commas to URL possible?

(copy & paste from several comments)
It might be due to URLSearchParams and its toString method implementation - but we can’t know, because you have not shown us what that actually is. If that is not deliberately encoding the comma, and the browser simply does it automatically - then there’s little you can do about that.
If newRelativePathQuery contains the encoded versions already, maybe they could be replaced back to normal commas. But if history.pushState does it, then “other ways” to create the URL itself won’t help you much.
Since a debug output showed that newRelativePathQuery contains the encoded commas already, you can try and replace them back to commas, and see if that “survives” being pushed to the history then.

It's a little hacky, but here's one solution. Let's say we want to use URL's searchParams.set() to set ids=1,2,3,4 in our query string.
If you just do url.searchParams.set("ids", "1,2,3,4"), the URL will have ids=1%2C2%2C3%2C4. To avoid that encoding, first set ids=LIST_OF_IDS_PLACEHOLDER, get the URL as a string, and then replace LIST_OF_IDS_PLACEHOLDER with 1,2,3,4, like this:
const myList = [1,2,3,4],
url = new URL(document.location.href); // or however you get your URL object
url.searchParams.set("ids", "LIST_OF_IDS_PLACEHOLDER");
const newUrlString = url.toString().replace("LIST_OF_IDS_PLACEHOLDER", ids.join(','));
console.log(newUrlString); // this will include: ids=1,2,3,4

Get data from URL address

I have a website like below:
localhost:3000/D129/1
D129 is a document name which changes and 1 is section within a document.
Those two values change depends on what user selects.
How do I just extract D129 part from the URL using javascript?

window.location.pathname.match(/\/([a-zA-Z\d]*)/)[1]
^ that should get you the 1st string after the slash
var path = "localhost:3000/D129/1";
alert(path.match(/\/([a-zA-Z\d]*)/)[1])

You can use .split() and [1]:
a = "localhost:3000/D129/1";
a = a.split("/");
alert(a[1]);
This works if your URLs always have the same format. Better to use RegEx. Wanted to answer in simple code. And if you have it with http:// or something, then:
a = "http://localhost:3000/D129/1";
a = a.split("/");
alert(a[3]);
ps: For the RegEx version, see Tuvia's answer.

Passing in a key results in extra characters in my firebase URL, how do I remove them?

When placing the "key" variable inside of this string, it displays 'simplelogin%3A5' instead of 'simplelogin:5'. Is there a way to just pass in the latter?
var populateTasks = function(date, key){
$scope.ref = new Firebase("https://myfirebase.firebaseio.com/users/"+key+"/tasks");
};
results in: https://myfirebase.firebaseio.com/users/simplelogin%3A5/tasks
I need: https://myfirebase.firebaseio.com/users/simplelogin:5/tasks

var uri = "//what you need to convert";
var uri_dec = decodeURIComponent(uri);
var res = uri_dec;

Where does the value of key come from? If you get it from a URL, it makes sense that you see %3A.
A : has a special meaning in a URL, so it is escaped. And the URL escape sequence for a : is %3A.
To convert the %3A back to : you simply unescape it like this:
unescape(key)
Or use decodeURIComponent, which in this case accomplishes the same. The best way to decode the value depends on why it was encoded in the first place, hence my initial question.

Have you tried trimming key before concatenating it to the URL?
key = key.trim();

how retrieve the last portion of the url using regex or jquery

I have a url
/stars/planets/usa/en/universe/planet_stars.html
I need to get the planets_stars.html. How do I get that last portion alone?

Neither jQuery nor regex necessary here:
url.split('/').pop();

Using pure javascript:
var url = "/stars/planets/usa/en/universe/planet_stars.html";
var page = url.substring(url.lastIndexOf("/")+1);
Edit: second parameter of substring method optional and not required in this case. So, I removed it.

or just JavaScript.
var parts = url.split('/');
var lastpart = parts[parts.length -1];
You can use pop() as jmar said too, just remember that removes it from the array when you do it, so you can't use the pop method twice on the same array and get the same value.

How do I trim/strip the URL down to the page name?

How would I go about trimming/stripping the URL down to the page name...
So: http://www.BurtReynoldsMustache.com/whatever/whoever/apage.html
Would become: apage.html
Any ideas?

you do not need jquery:
var url = window.location.href;
var page = url.substring(url.lastIndexOf('/') + 1);
Edit: a good point of the possible query string:
// it might be from browser & / anywhere else
var url = window.location.href;
url = url.split('#').pop().split('?').pop();
var page = url.substring(url.lastIndexOf('/') + 1);
ok, if the location object is available, use pathname gives better result as show below, however, a url can be a string or something directly from text field or span/label. So above solution should have its place.

With location and any link (<a>) elements on the page, you get a load of properties that give you specific parts of the URL: protocol, host, port, pathname, search and hash.
You should always use these properties to extract parts of the URL in preference to hacking about with href and probably getting it wrong for corner cases. For example, every solution posted here so far will fail if a ?query or #fragment is present. The answers from Rob and digitalFresh attempt to cope with them, but will still fail if a / character is present in the query string or fragment (which is valid).
Instead, simply:
var pagename= location.pathname.split('/').pop();

Most of the solutions here are not taking advantage of the window.location object. The location object has this wonderful thing called pathname which returns just the path, no query string, host, protocol, hash, etc.
var mypage = window.location.pathname.split("/").pop();

You could do something like this:
document.location.href.split('/').pop();
Edit: you probably want to get rid of the query string if there is one also:
document.location.href.split('/').pop().split('?').shift();
Edit 2: this will also ignore an anchor in the url if there is one
document.location.href.split('/').pop().split(/\?|#/).shift();

This should also exclude query and hash values.
var path = location.href;
path = path.substring(path.lastIndexOf("/") + 1);
path = path.split("?")[0].split("#")[0];
console.debug(path);

Haven't tested so compeltely guessed, but I'm sure something like this will do :-)
var url = 'http://www.BurtReynoldsMustache.com/whatever/whoever/apage.html';
var page = url.split('/');
alert(page[page.length-1]);​
EDIT Tested under jsfiddle and it was wrong, the above code should now work :-)