Array.prototype.concat

/**
 * @template T
 * @param {...(T | Array<T>)} items
 * @return {Array<T>}
 */
Array.prototype.myConcat = function (...items) {
  const result = new Array(this.length);
  for (let i = 0; i < this.length; i++) {
    if (Object.prototype.hasOwnProperty.call(this, i)) {
      result[i] = this[i];
    }
  }
  for (const item of items) {
    if (Array.isArray(item)) {
      for (let i = 0; i < item.length; i++) {
        if (Object.prototype.hasOwnProperty.call(item, i)) {
          result.push(item[i]);
        }
      }
    } else {
      result.push(item);
    }
  }
  return result;
};