Don't Just String Append to a Querystring

Something I've been guilty of doing in the past, but am trying to make up for, is using string concatenation with URLs to append parameters to a querystring.

For instance, let's take the following Go code:

package main

import (
	"fmt"
)

func main() {
  u := "https://www.jvt.me/posts/2022/01/20/dont-append-querystring/"
  fmt.Println(u + "?utm_medium=code_example");
}

In this case, the URL will be correctly output as:

https://www.jvt.me/posts/2022/01/20/dont-append-querystring/?utm_medium=code_example

It's something that is generally safe, but you're going to hit "edge cases" more often than not, such as a URL already having a querystring on it:

http://example.com/page?p=132
# when concatenated, it is *not* a valid URL:
http://example.com/page?p=132?utm_medium=code_example

Or we may have an empty querystring:

http://example.com/page?
# when concatenated, it is *not* a valid URL:
http://example.com/page??utm_medium=code_example

Or we may have a fragment on the URL:

http://example.com/redirect#code=foo
# when concatenated, it is *not* a valid querystring, as it's part of the fragment:
http://example.com/redirect#code=foo?utm_medium=code_example

Or even if the URL already has a query parameter of that name, and we don't specify what we want the behaviour to be, which can cause confusion between URL parsing libraries:

http://example.com/page?utm_medium=existing
# when concatenated
http://example.com/page?utm_medium=existing&utm_medium=code_example

So what should we do instead?

We should treat the URL as a URL, not an arbitrary string, and then append/replace the parameter according to what we want to do.