Custom-concat()-Array-Method.js

/*
  TL;DR

  Create a customConcat() method with the same functionality as Array.prototype.concat().

  SYNTAX

  var new_array = old_array.concat([value1[, value2[, ...[, valueN]]]])

  Parameter valueN: Arrays and/or values to concatenate into a new array. If valueN is undefined, concat returns a shallow copy of the existing array on which it is called. See the description below for more details.

  DESCRIPTION

  The concat method creates a new array consisting of the elements in the object on which it is called, followed in order by, for each argument, the elements of that argument (if the argument is an array) or the argument itself (if the argument is not an array). It does not recurse into nested array arguments.

  The concat method does not alter this or any of the arrays provided as arguments but instead returns a shallow copy that contains copies of the same elements combined from the original arrays. Elements of the original arrays are copied into the new array as follows:

  Object references (and not the actual object): concat copies object references into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays. This includes elements of array arguments that are also arrays.

  Data types such as strings, numbers and booleans (not String, Number, and Boolean objects): concat copies the values of strings and numbers into the new array.
*/

// Answer:

Array.prototype.customConcat = function (...elements) {
  if (!elements) return [...this];
  const result = [...this];
  for (let element of elements) {
    result.push(...element);
  }
  return result;
};

// BigO: O(n * k) where n amount of elements and k is length of these elements