en.javascript.info/9-regular-expressions/04-regexp-anchors/1-start-end/solution.md

An empty string is the only match: it starts and immediately finishes.

The task once again demonstrates that anchors are not characters, but tests.

The string is empty "". The engine first matches the pattern:^ (input start), yes it's there, and then immediately the end pattern:$, it's here too. So there's a match.