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How to ignore invalid URL parameters?

Before I go on, let me say that I've looked through a number of threads already and can't find an answer that works for me.
Basically, I've built a custom link shortener and I'm using URLSearchParams to pull the URL to be shortened and the custom slug from the URL search query as follows:
var e = window.location.search;
const urlParams = new URLSearchParams(e);
const url = urlParams.get("url");
const slug = urlParams.get("slug");
Where the format for a query is: ?url=https://google.com&slug=customslug
After the parameters are handled, the URL string is treated with trim() to remove any whitespace. The final output is encoded with encodeURIComponent() when the API I'm using (https://short.io) is called.
However, I want to be able to pass URLs with &, like so: ?url=https://google.com/&testing&slug=customslug. My ideal solution would simply treat any & that isn't part of the &slug parameter as a part of the URL contained within the &url parameter. Currently, the & character is ignored if it isn't attached to a valid parameter (url or slug).
I have tried encoding the query input using encodeURIComponent(), but that results in a failure to pick up on either defined parameter. I have also tried splitting the input using split("&slug",1), but that results in an array and I cannot pass arrays to the Short.io API.
Any suggestions?

You should use the URL Encoded ampersand symbol %26.
var e = "?url=https://google.com/%26testing&slug=customslug";
const urlParams = new URLSearchParams(e);
const url = urlParams.get("url");
const slug = urlParams.get("slug");
console.log(url);
console.log(slug);

I solved my issue by building off of #CherryDT's comment about using window.location.hash to get my URL string. Ultimately, I chose to forgo the idea of a slug in the address bar, since it would cause further problems with my script.
While this solution is only applicable for my purposes, I'm detailing the solution because it functions as a workaround for the issue of not being able to encode a passed URL string from the address bar. Might be useful to someone, someday.
var e = window.location.href.replace(window.location.hash, '');
if (e.endsWith("?") === true) {
var url = window.location.hash.substr(1);
if (url === "") {
// Error code
} else {
console.log("A URL to be shortened was provided via hash.");
// Pass url to rest of script
}
}

Java Script: Passing variables between html pages

I am practicing making an e-commerce site. I am struggling passing my javascript variable between the shopping cart page to the checkout page. I tried following this example:
page one:
<a href="example2.html?myVar1=42&myVar2=66" >LINK</a>
page 2:
var qs = new Querystring();
var v1 = qs.get("myVar1");
var v2 = qs.get("myVar2");
I could not get it to work because my webpage threw an error about not recognizing QueryString, so I am currently trying like this by using window.localStorage as follows:
Page 01:
<button class="checkout" type="button" onClick="window.location.href='checkout.html'; window.localStorage.setItem("total",total)">Checkout</button>
Page 2:
var name = window.localStorage.getItem("total");
Currently, I am throwing a console.log(total) on the 2nd page but it returns "null" each time. Any idea why it's not catching the value from the first page? Please help.
If you have suggestions or a solution for either method, it would be much appreciated. I feel like I'm really close with the 2nd method, I may just be missing something on the page 1 portion, thanks in advance.

To fix your first method (passing variables between pages via a query string), you'd probably want something like:
// page one:
<a href="example2.html?myVar1=42&myVar2=66" >LINK</a>
// page 2:
var query = window.location.search;
if( typeof query !== 'undefined' ) {
var params = {},
parts
;
// take off the ? and split into groups of key=value strings
query = query.replace('?','').split('&');
// split key=value strings into a usable object
for(var i=0;i<query.length;i++){
parts = query[i].split('=');
params[ parts[0] ] = parts[1];
}
// now access your variables like so:
params.myVar1 // is now "42" (a string)
params.myVar2 // "66"
}

Change the pathname in the URL of a link while keeping the querystring constant

From a page with the following URL, http://example.com/foo.html?query=1&other=2, I want to create a link to http://example.com/bar.html?query=1&other=2. How do I do that without explicitly saving and reloading all the query strings.
I need this to easily link from an iframe version of a page (embed.html?query) to the full page (index.html?query).

I would have recommended using the Location object's search method (available at document.location or window.location) to pull out the parameters, then modify the rest of the URL, but that API is apparently specific to Firefox.
I would simplify #DMortensen's answer by just splitting on the first ?, then modifying the first part (which will be the URL's path portion only), and reapplying the second part.
If you need to parse the parameters, I recommend the jQuery plugin Query Parameter Parser: one call to $.parseQuery(s) will pull out an object of all the keys & values.

It can be finicky, but you could split the URI on '?' and then loop through the 2nd element of that array to grab the key/val pairs if you need to evaluate each pair (using '&' as a delimiter). The obvious weakness in this would be if there are additional '?' or '&' used in the URI.
Something like this maybe? (pseudocode-ish)
var URI = document.URL;
var qs = URI.split('?');
var keyvalpair = qs[1].split('&');
var reconstructedURI = '&' + keyvalpair;
for(var i = 0; i< keyvalpair.length; i++){
var key = keyvalpair[i].split('=')[0];
var val = keyvalpair[i].split('=')[1];
}

Thank you for all the answers. I tried the following and it works.
function gotoFullSite() {
var search = window.location.search;
window.open("http://example.com/"+search)
}
$('#clickable').click(gotoFullSite);
and then use <a id = "clickable" href="#"></a>. When I click the link, it opens the proper website with all the query parameters in a new tab. (I need a new tab to break out of an iframe.)

How to Execute Javascript Code Using Variable in URL

I'm really new to Javascript and I'm having some trouble understanding how to get the following to work. My goal is to have a certain Javascript action execute when a page loads and a variable added to the end of the URL would trigger which Javascript action to execute. The URL of the page that I'm looking to implement this on is http://www.morgantoolandsupply.com/catalog.php. Each of the "+expand" buttons, which are Javascript driven, drop-down a certain area of the page. Ultimately, I would like to be able to create a URL that would automatically drop-down a certain category when the page loads. Could anybody explain to me the process to do this? Thanks in advance for any help!

You have to parse the URL somewhat "manually" since the parameters in the url aren't automatically passed to javascript, like they are in server-side scripting (via $_GET in PHP, for instance)
One way is to the use the URL fragment identifier, i.e. the "#something" bit that can go at the end. This is probably the neatest way of doing it, since the fragment isn't sent to the server, so it won't be confused with any other parameters
// window.location.hash is the fragment i.e. "#foo" in "example.com/page?blah=blah#foo"
if( window.location.hash ) {
// do something with the value of window.location.hash. First, to get rid of the "#"
// at the beginning, do this;
var value = window.location.hash.replace(/^#/,'');
// then, if for example value is "1", you can call
toggle2('toggle' + value , 'displayText' + value);
}
The URL "http://www.morgantoolandsupply.com/catalog.php#1" would thus automatically expand the "toggle1" element.
Alternatively, you can use a normal GET parameter (i.e. "?foo=bar")
var parameter = window.location.search.match(/\bexpand=([^&]+)/i);
if( parameter && parameter[1]) {
// do something with parameter[1], which is the value of the "expand" parameter
// I.e. if parameter[1] is "1", you could call
toggle2('toggle' + parameter[1] , 'displayText' + parameter[1]);
}
window.location.search contains the parameters, i.e. everything from the question mark to the end or to the URL fragment. If given the URL "example.com/page.php?expand=foo", the parameter[1] would equal "foo". So the URL "http://www.morgantoolandsupply.com/catalog.php?expand=1" would expand the "toggle1" element.
I'd perhaps go for something more descriptive than just a number in the URL, like, say use the title of the dropdown instead (so "#abrasives" or "expand=abrasives" instead of "#1" or "expand=1"), but that would require a little tweaking of your existing page, so leave that for later

You've already got the function to call: toggle2(), which takes two parameters that happen to be identical for all categories except for a number at the end. So create a URL that includes that number: http://www.morgantoolandsupply.com/catalog.php#cat=4
Then find that number in location.hash using a regular expression. This one is robust enough to handle multiple url parameters, should you decide to use them in the future: /[\#&]cat=(\d+)/. But, if you expect to never add anything else to the url, you could use a very simple one like /(\d+)/.
Once you've got the number, it's a simple matter of using that number to create your two parameters and calling toggle2().
This should work:
window.onload = function() {
if (/[\#&]cat=(\d+)/.test(location.hash)) {
var cat = parseInt(RegExp.$1);
if (cat > 0 && cat < 13) {
toggle2("toggle"+cat, "displayText"+cat);
}
}
}

Not a complete answer ("Give a man a fish" and all that), but you can start with something along these lines:
// entire URL
var fullURL = window.location.href;
// search string (from "?" onwards in, e.g., "www.test.com?something=123")
var queryString = window.location.search;
if (queryString.indexOf("someParameter") != -1) {
// do something
}
More info on window.location is available from the Mozilla Developer Network.
Having said that, given that you're talking about a PHP page why don't you use some server-side PHP to achieve the same result?

Get URL parameters and insert into form action url with javascript

I have an html page that people access using an affiliate link, so it has affiliate code in the url (http://www.site.com?cmpid=1234&aid=123). I want to add the cmpid and the aid to the form action url (so when submitted, it submits to the url /form.aspx but adds the cmpid and aid to the end, ie: form.aspx?cmpid=1234&aid=123).
I have no other option than javascript. I can't make this page php or aspx.

window.onload = function() {
var frm = document.forms[0];
frm.action = frm.action + location.search;
}

You can get access to the query string by location.search. Then you should be able to append it onto the form's action directly, something like the following:
// Get a reference to the form however, such as document.getElementById
var form = ...;
// Get the query string, stripping off the question mark (not strictly necessary
// as it gets added back on below but makes manipulation much easier)
var query = location.search.substring(1);
// Highly recommended that you validate parameters for sanity before blindly appending
// Now append them
var existingAction = form.getAttribute("action");
form.setAttribute("action", existingAction + '?' + query);
By the way, I've found that modifying the query string of the form's action directly can be hit-and-miss - I think in particular, IE will trim off any query if you're POSTing the results. (This was a while ago and I can't remember the exact combination of factors, but suffice to say it's not a great idea).
Thus you may want to do this by dynamically creating child elements of the <form> that are hidden inputs, to encode the desired name-value pairs from the query.