Flatten the children of two or more NodeLists into an array

flatChild.js

  // Magic code to flatten the children of two or more nodelists.
  // [...sides] converts the two nodelists into two arrays. this is then
  // mapped to get the children. [...element.children] gets the children of each
  // side, but in the form of an array. This is done so that there will be n arrays
  // that is composed of the children, this is then flattened
  const getChildren = (sides) => [...sides].map((element) => [...element.children]).flat();