Deep version of Object.assign.

objectAssignDeep.js

const objectAssignDeep = (...objs) => objs.reduce((acc, obj) => Object.assign(acc, ...Object.keys(obj).map(key => {
  if (acc[key] instanceof Object && obj[key] instanceof Object) {
    return { [key]: objectAssignDeep({}, acc[key], obj[key]) };
  }                                                                       
  return { [key]: obj[key] };
})));

Example:

objectAssignDeep({}, {a: {b: 3, c:5}}, {a: {c: 6, d:7}}) // { a: { b: 3, c: 6, d: 7 } }

Note that this function has the same characteristic as Object.assign, the first object will be mutated and the rest remains.

obj1 = {};
obj2 = {a: {b: 3, c:5}};
obj3 = {a: {c: {}, d:7}};
objectAssignDeep(obj1, obj2, obj3); // { a: { b: 3, c: {}, d: 7 } }

obj1; // { a: { b: 3, c: {}, d: 7 } }
// obj2, obj3 unchanged