Object.merge(target, ...sources) : Deep merge for JS Objects

merge.js

/** https://gist.github.com/Juknum/6750c0ec3a4249359b5be0641185f917 => Object.isObject(object) */

Object.defineProperty(Object.prototype, 'merge', {
  /**
   * @param {Object} target 
   * @param  {...Object} sources 
   */
  value: (target, ...sources) => {
    if (!sources.length) return target
    const source = sources.shift()

    if (Object.isObject(target) && Object.isObject(source)) {
      for (const key in source) {
        if (Object.isObject(source[key])) {
          if (!target[key]) Object.assign(target, { [key]: {} })
          Object.merge(target[key], source[key])
        }

        else Object.assign(target, { [key]: source[key] })
      }
    }

    return Object.merge(target, ...sources)
  }
})

/**
 * Example:
 * const target = { a: { b: 'overwritten' }, b: { m: 2 }, c: { n: { y: 'a' } } }
 * const obj = { a: 0, b: { n: 1 }, c: { n: { z: 'b' } } }
 * const obj2 = { d: 5 }
 * console.log(Object.merge(target, obj, obj2))
 * => expected output: { a: 0, b: { m: 2, n: 1 }, c: { n: { y: 'a', z: 'b' } } }
 */