A deep merge of options or configuration values

merge.js

export const merge = (target, ...sources) => {
  target = { ...target }
  if (!sources.length) return target
  const source = sources.shift()
  for(const key in source) {
    const value = target[key]
    const isObject = value 
      && typeof value == 'object'
      && !(value instanceof Date) 
      && !(value instanceof RegExp)
    target[key] = isObject
      ? merge(target[key], source[key])
      : target[key] ?? source[key]
  }
  return Array.isArray(target)
    // for my configuration/option purposes, treating arrays as sets is more useful,
    // but it's not neceesary, if you don't mind duplicate values.
    ? Array.from(new Set([...source, ...target]))
    : { ...source, ...target }
}