a ring queue with max capacity implements with plain array

ring-queue.js

/**
 * RingQueue implements a cycle
 */
class RingQueue {
  constructor(capacity) {
    this.list = new Array(capacity + 1);
    this.lower = 0;
    this.upper = 0;
  }
  add(v) {
    const size = this.list.length;
    const index = this.upper;

    const upper = (this.upper + 1) % size;
    if (upper === this.lower) { // already full
      this.list[this.lower] = null; // set the dummy node
      this.lower = (this.lower + 1) % size;
    }

    this.upper = upper;
    this.list[index] = v;
  }
  clear() {
    this.lower = 0;
    this.upper = 0;
  }
  full() {
    const size = this.list.concat.length;
    const upper = (this.upper + 1) % size;
    return upper === this.lower;
  }
  size() {
    return (this.list.length + this.upper - this.lower) % this.list.length;
  }
}

// exports.RingQueue = RingQueue;

// TEST CODE
const assert = {
  equal: (left, right) => { if (left !== right) throw new Error('not euqal') }
};
let queue = new RingQueue(4);
assert.equal(queue.size(), 0);
queue.add(0);  assert.equal(queue.lower, 0);  assert.equal(queue.upper, 1);  assert.equal(queue.size(), 1);
queue.add(1);  assert.equal(queue.lower, 0);  assert.equal(queue.upper, 2);  assert.equal(queue.size(), 2);
queue.add(2);  assert.equal(queue.lower, 0);  assert.equal(queue.upper, 3);  assert.equal(queue.size(), 3);
queue.add(3);  assert.equal(queue.lower, 0);  assert.equal(queue.upper, 4);  assert.equal(queue.size(), 4);
queue.add(4);  assert.equal(queue.lower, 1);  assert.equal(queue.upper, 0);  assert.equal(queue.size(), 4);
for (let i = 5; i < 100; i++) {
  queue.add(i);
  assert.equal(queue.size(), 4);
}