DDDD is an esoteric programming language whose only data structure is a single deque. It was created by User:Andrew3335 in 2021 and is intended to be used for golfing, although the restriction to printable ASCII limits its brevity.
N...} puts an element containing "Hello, World!" at the end of the deque. J looks at the back element and prints it.
i takes STDIN and creates a new element at the deque's top. K looks at the front element and prints it.
DDDD is Turing-complete as the instructions :()^ from Underload are in DDDD.