Deeply merge js objects

deepMerge.js

/**
 * Merges two arrays by concatenating their elements uniquely using `new Set(...)`.
 *
 * @param {Array} a1 - The first array to merge.
 * @param {Array} a2 - The second array to merge.
 * @returns {Array} A new array containing elements from both input arrays.
 */
const mergeArray = (a1, a2) => Array.from(new Set([...a1, ...a2]))

/**
 * Recursively merges two objects based on their types and structure.
 *
 * @param {*} o1 - The first object to merge.
 * @param {*} o2 - The second object to merge.
 * @returns {*} A new object that is a merge of the two input objects.
 * @throws {Error} Throws an error if either input is not an object.
 */
const merge = (o1, o2) => {
  if (typeof o1 === 'undefined') return o2

  if (typeof o1 !== typeof o2) return undefined

  if (Array.isArray(o1) && Array.isArray(o2)) return mergeArray(o1, o2)

  // They're some other type, prioritize the second object as there is no merge strategy
  if (typeof o1 !== 'object') return o2

  return Object.keys(o2).reduce((acc, k) => {
    const result = merge(o1[k], o2[k])
    return result ? { ...acc, [k]: result } : acc
  }, o1)
}

/**
 * Deeply merges two objects by recursively combining their properties.
 *
 * @param {Object} o1 - The first object to merge.
 * @param {Object} o2 - The second object to merge.
 * @returns {Object} A new object that is a deep merge of the two input objects.
 * @throws {Error} Throws an error if either input is not an object.
 */
const deepMerge = (o1, o2) => {
  if (typeof o1 !== 'object')
    throw new Error(`First argument should an object got: ${typeof o1}`)
  if (typeof o2 !== 'object')
    throw new Error(`Second argument should an object got: ${typeof o2}`)

  return merge(o1, o2)
}

/**
 * Deeply merges an array of objects into a single object, applying a recursive merge operation.
 *
 * @param {Array} arr - An array of objects to merge.
 * @returns {Object} A new object that is a deep merge of all objects in the input array.
 * @throws {Error} Throws an error if any element in the array is not an object.
 */
const deepMergeAll = (...[head, ...tail]) =>
  tail?.length ? tail.reduce((acc, o) => deepMerge(acc, o), head) : head