Deep merge with comparator to ensure null/undefined data values are patched

deep-merge-example.js

import { mergeWith, isNil } from "lodash"

var existing = {
  a: 1,
  b: 2,
  d: {
    a: 1,
    b: [],
    c: { test1: 123, test2: 321 }
  },
  f: 5,
  g: 123,
  i: 321,
  j: [1, 2]
}

var data = {
  b: 3,
  c: undefined,
  d: {
    b: { first: 'one', second: 'two' },
    c: { test2: 222 }
  },
  e: { one: 1, two: 2 },
  f: [],
  g: (void 0),
  h: /abc/g,
  i: null,
  j: [3, 4]
}

const res = mergeWith(
  existing,
  data,
  (a, b) => isNil(b) ? null : undefined)
)

/*
{
  a: 1, 
  b: 3, 
  d:  
   { a: 1, 
     b: [ first: 'one', second: 'two' ], 
     c: { test1: 123, test2: 222 } }, 
  f: [], 
  g: 123, 
  i: null, 
  j: [ 3, 4 ], 
  c: null, 
  e: { one: 1, two: 2 }, 
  h: /abc/g
}
 */