class Pipeline {
  #value;
  #log = [];

  constructor(value) {
    this.#value = value;
  }

  map(fn) {
    this.#value = fn(this.#value);
    this.#log.push(`map:${this.#value}`);
    return this;
  }

  filter(fn) {
    if (Array.isArray(this.#value)) {
      this.#value = this.#value.filter(fn);
      this.#log.push(`filter:${this.#value}`);
    }
    return this;
  }

  reduce(fn, init) {
    this.#value = this.#value.reduce(fn, init);
    this.#log.push(`reduce:${this.#value}`);
    return this;
  }

  result() {
    console.log(this.#log.join(' | '));
    return this.#value;
  }
}

const output = new Pipeline([1, 2, 3, 4, 5, 6])
  .filter(x => x % 2 === 0)
  .map(arr => arr.map(x => x ** 2))
  .reduce((acc, x) => acc + x, 0)
  .result();

console.log(output);