source: node_modules/ramda/es/internal/_concat.js

/**
 * Private `concat` function to merge two array-like objects.
 *
 * @private
 * @param {Array|Arguments} [set1=[]] An array-like object.
 * @param {Array|Arguments} [set2=[]] An array-like object.
 * @return {Array} A new, merged array.
 * @example
 *
 *      _concat([4, 5, 6], [1, 2, 3]); //=> [4, 5, 6, 1, 2, 3]
 */
export default function _concat(set1, set2) {
  set1 = set1 || [];
  set2 = set2 || [];
  var idx;
  var len1 = set1.length;
  var len2 = set2.length;
  var result = [];
  idx = 0;

  while (idx < len1) {
    result[result.length] = set1[idx];
    idx += 1;
  }

  idx = 0;

  while (idx < len2) {
    result[result.length] = set2[idx];
    idx += 1;
  }

  return result;
}