Why is the argument undefined, though?

I do not know why.
It may be a jsdom specific issue.

Here's a simple way to reproduce the issue.

• Save a clone of jsdom to your local PC.
• Copy the latest source of https://raw.githubusercontent.com/web-platform-tests/wpt/master/resources/testharness.js and paste it into local jsdom/test/web-platform-tests/tests/resources/testharness.js.
• Run npm test.

I took a brief look at the source.

In L4514:

    function render(template, substitutions, output_document)

render() accepts 3 arguments, and the 2nd is the substitutions.

However, for example, in L4290, only 1 argument is specified.
So substitutions will be undefined.

            const asserts_output = render(
                ["details", {},
                    ["summary", {}, "Asserts run"],
                    ["table", {}, ""] ]);

By the way, the previous PR had an array as the default value, which was a mistake.
substitutions[components[i + 1]] is a property of substitutions.

substitutions = {
  [components[i + 1]]: foo
}

So, fixed default value to substitutions = {}.

Ready for review now.
Please take a look.

Some notes about the issue:

function substitute_single(template, substitutions) accepts two arguments, but the second subustitutions may be undefined.
See #issuecomment-2203051749 above.

On the other hand, in the internal function function do_substitution(input), it is assuming that substitutions always exists.
Or if substitutions is undefined, the length of components would always be 1.

Therefore, if substitutions is undefined and components.length is greater than 1, exception raises at substitutions[components[i + 1]];.