Adding more than one GET value in the href

Question

I got this HTML with several unordered lists.

<ul>
    <h3>Staff</h3>
    <li><a href="?staff=Mike">Mike</a></li>
    <li><a href="?staff=John">John</a></li>
    <li><a href="?staff=Kyle">Kyle</a></li>
</ul>
<ul>
    <h3>Category</h3>
    <li><a href="?category=Food">Food</a></li>
    <li><a href="?category=Sports">Sports</a></li>
    <li><a href="?category=News">News</a></li>
    <li><a href="?category=Games">Games</a></li>
    <li><a href="?category=GIFs">GIFs</a></li>
</ul>

As can be seen, all of them contains with params values in each of its hrefs. My question is: using Javascript, how can I append a new value in a param in the href if one of them are actives in the URL.

IE: The URL is www.site.com/?category=Food.

So the lists should be valued as:

<ul>
    <h3>Staff</h3>
    <li><a href="?category=Food&staff=Mike">Mike</a></li>
    <li><a href="?category=Food&staff=John">John</a></li>
    <li><a href="?category=Food&staff=Kyle">Kyle</a></li>
</ul>
<ul>
    <h3>Category</h3>
    <li><a href="?category=Food">Food</a></li>
    <li><a href="?category=Food,Sports">Sports</a></li>
    <li><a href="?category=Food,News">News</a></li>
    <li><a href="?category=Food,Games">Games</a></li>
    <li><a href="?category=Food,GIFs">GIFs</a></li>
</ul>

Answer 1

What you can do is

1. Parse the current query string into an URLSearchParams object
2. For each <a> element, parse the href into an URL object
3. Iterate the query params from step #1 and...
   1. Check if the param already exists
   2. If it does not, simply append it
   3. If it does, then parse the current list into a Set, add the value from the query string, then write the value back into the URL
4. Write the URL into the <a> href property

//const query = location.search
const query = "?category=Food" // this is for the snippet

// Parse any CSV values into arrays and store in a `Map`
const params = new Map()
for (let [ key, val ] of (new URLSearchParams(query))) {
  params.set(key, val.split(","))
}

const links = document.querySelectorAll("ul li a[href]")

links.forEach(link => {
  const url = new URL(link.href)
  
  // Loop the page query params
  for (let [ param, values ] of params) {
    if (url.searchParams.has(param)) {
      // Split the current value on "," and parse to a `Set`
      const linkParams = new Set(url.searchParams.get(param).split(","))
      
      // Add the query string value then write the param back into the URL
      values.forEach(v => linkParams.add(v))
            
      url.searchParams.set(param, [...linkParams].join(","))
    } else {
      // Simply append the param value 
      url.searchParams.append(param, values.join(","))
    }
  }
  link.href = url
})

/* This just makes it easy to see the URLs in the demo */
a[href]:after {
  content: " - " attr(href);
  font-size: .8;
  color: grey;
}

<ul>
    <h3>Staff</h3>
    <li><a href="?staff=Mike">Mike</a></li>
    <li><a href="?staff=John">John</a></li>
    <li><a href="?staff=Kyle">Kyle</a></li>
</ul>
<ul>
    <h3>Category</h3>
    <li><a href="?category=Food">Food</a></li>
    <li><a href="?category=Sports">Sports</a></li>
    <li><a href="?category=News">News</a></li>
    <li><a href="?category=Games">Games</a></li>
    <li><a href="?category=GIFs">GIFs</a></li>
</ul>