Deep and shallow merge in javascript

Question

What is the difference between deep and shallow merge of objects in javascript? As far as I understand, deep merge recursively copies all the source object enumerable properties

Answer 1

In a shallow merge, the properties of the first object are overwritten with the same property values of the second object.

Lets look at an example. Setup:

var obj1 = {
  foo: {
    prop1: 42,
  },
};

var obj2 = {
  foo: {
    prop2: 21,
  },
  bar: {
    prop3: 10,
  },
};

Shallow:

var result = {
  foo: {          // `foo` got overwritten with the value of `obj2`
    prop2: 21,
  },
  bar: {
    prop3: 10,
  },
};

Deep:

var result = {
  foo: {
    prop1: 42,
    prop2: 21,    // `obj2.foo` got merged into `obj1.foo`.
  },
  bar: {
    prop3: 10,
  },
};