Array.prototype._reduce_ = function (callback, initialValue) {
  if (this === 'undefined') {
    throw new TypeError('this is null or undefined');
  }
  if (typeof callback !== 'function') {
    throw new TypeError(`${callback} is not a function`);
  }
  const O = Object(this);
  const len = this.length >>> 0;
  let accumulator = initialValue;
  let k = 0;
  // if no initial value, the first item of array will be setted for initial value
  if (accumulator === undefined) {
    while (k < len && !(k in O)) {
      k++;
    }
    // not found initial value
    if (k >= len) {
      throw new TypeError('Reduce of empty array with no initial value');
    }
    accumulator = O[k++];
  }
  while (k < len) {
    if (k in O) {
      accumulator = callback.call(undefined, accumulator, O[k], k, O);
    }
    k++;
  }
  return accumulator;
};

// test
const arr = [1, 2, 3, 4, 5];
const newArr = arr._reduce_((acc, cur) => {
  acc.push({
    key: cur
  });
  return acc;
}, []);

console.log(newArr);