A circular buffer that keeps the last N entries

A long-running tab that keeps every console line, every websocket message or every frame timing will eventually eat all the memory it can. A circular buffer fixes the size up front: once it is full, each new write overwrites the oldest entry.

Implement a CircularBuffer class. new CircularBuffer(capacity) takes a positive integer capacity and starts empty. write(value) always stores the value, and returns true if there was free space or false if it overwrote the oldest entry. read() removes and returns the oldest stored value, or undefined when the buffer is empty. size() returns how many values are currently stored, never more than the capacity. isFull() returns true when size() equals the capacity. toArray() returns the stored values as a plain array from oldest to newest, and [] when empty.

const b = new CircularBuffer(3);
b.write(1); b.write(2); b.write(3);
b.write(4);     // false — overwrote the oldest
b.toArray();    // [2, 3, 4]

const b = new CircularBuffer(2);
b.write("a"); b.write("b");
b.read();  // "a" — oldest first
b.read();  // "b"
b.read();  // undefined — empty