That means you’re not yet being affected by it.
100%. TDD is just not practicably applicable to a lot of scenarios and I wish evangelists were clearer on that detail.
Not sure why you focus on arrays for deep copying. Deep copying objects is a problem in many languages and brings some challenges with itself that make it almost always necessary to delegate it to a library.
Client side logic is contained in markup, no custom JavaScript. Need server-side-rendering logic to return new markup to update a chunk of the page.
That means you’re not yet being affected by it.