npm install deep-extend

var deepExtend = require('deep-extend');
var obj1 = {
    a: 1,
    b: 2,
    d: {
        a: 1,
        b: [],
        c: { test1: 123, test2: 321 }
    },
    f: 5,
    g: 123
};
var obj2 = {
    b: 3,
    c: 5,
    d: {
        b: { first: 'one', second: 'two' },
        c: { test2: 222 }
    },
    e: { one: 1, two: 2 },
    f: [],
    g: (void 0)
};

deepExtend(obj1, obj2);

console.log(obj1);
/*
    { a: 1,
      b: 3,
      d:
       { a: 1,
         b: { first: 'one', second: 'two' },
         c: { test1: 123, test2: 222 } },
      f: [],
      c: 5,
      e: { one: 1, two: 2 },
      g: undefined }
*/