Dynamic array

// A teaching model of a growing array. In real code, use your language's own:
// a TypeScript array, a Go slice, a Rust Vec.
class GrowingArray {
  slots: (number | undefined)[] = new Array(4).fill(undefined);
  length = 0;
  /** Entries copied, shifted or written: the work this lesson counts. */
  moves = 0;

  get capacity(): number {
    return this.slots.length;
  }

  append(value: number): void {
    if (this.length === this.capacity) {
      // Full: copy every entry into a buffer twice the size, then use it.
      const oldSlots = this.slots;
      const newSlots: (number | undefined)[] = new Array(oldSlots.length * 2).fill(undefined);
      for (let i = 0; i < this.length; i++) {
        newSlots[i] = oldSlots[i];
        this.moves++;
      }
      this.slots = newSlots;
    }
    this.slots[this.length] = value;
    this.length++;
    this.moves++;
  }

  get(index: number): number | undefined {
    return index >= 0 && index < this.length ? this.slots[index] : undefined;
  }

  remove(index: number): number | undefined {
    if (index < 0 || index >= this.length) return undefined;
    const removed = this.slots[index];
    // Keep the order: move every later entry one place earlier.
    for (let i = index + 1; i < this.length; i++) {
      this.slots[i - 1] = this.slots[i];
      this.moves++;
    }
    this.length--;
    this.slots[this.length] = undefined;
    return removed;
  }

  values(): number[] {
    return this.slots.slice(0, this.length) as number[];
  }
}

const results = new GrowingArray();
for (const bib of [12, 7, 31, 4]) results.append(bib);

const beforeFifth = results.moves;
results.append(19);
const fifthAppend = results.moves - beforeFifth;
console.log(`five: ${results.values().join(", ")} | capacity ${results.capacity} | fifth append ${fifthAppend} moves`);

const beforeRemove = results.moves;
const removed = results.remove(1);
const removal = results.moves - beforeRemove;
console.log(`removed ${removed}: ${results.values().join(", ")} | capacity ${results.capacity} | ${removal} moves`);
console.log(`index 1 now: ${results.get(1)}`);