Insert new item into a Map object after specified key.

insertAfterKey.js

/**
 * Insert 'insert' 2-D array after 'after' key in 'map'
 * WARNING: THIS MUTATES THE ORIGINAL Map!
 * @param {Map} map - Map to insert into
 * @param {any} after - key to insert after
 * @param {[any, any]} insert - 2-D array for entry to insert
 * @returns {Map} - returns mutated 'map' passed as first argument
 */
function insertAfterKey(map, after, insert){
  const [insertKey, insertValue] = insert;
  // Copy to temp Map before clearing
  const tmp = new Map(map);
  // Remove all entries, but keep the reference
  map.clear();
  // Iterate 'tmp' and copy entries back to original reference
  for (const [key, value] of tmp) {
    map.set(key, value);
    // If the current key matches 'after'...
    if (key === after) {
      // ...insert the new item
      map.set(insertKey, insertValue);
    }
  }
  // If item is not yet inserted (there's no key that matches 'after')...
  if (!map.has(insertKey)) {
    // Insert item at the end
    map.set(insertKey, insertValue);
  }
  // Return same Map reference that was passed to the function
  return map;
}