jsbin.kikuf.js

console.clear();

var dest = {
  foo : {
    b1 : "b1 value",
    b2 : "b2 value"
  },
  baz : {
    q1 : "q1 value"
  },
  mofo : "mofo value"
};

var src = { 
  foo : { 
    b1: "overwritten b1", 
    b3: "b3 value"
  },
  mofo : "overwritten mofo"
};


var assigned = _.clone(dest);
_.assign(assigned,src);
console.log("assign:", assigned);

var merged = _.clone(dest);
_.merge(merged,src);
console.log("merge:", merged);

var defaulted = _.clone(dest);
_.defaults(defaulted,src);
console.log("defaults:", defaulted);

log.txt

"assign:"
[object Object] {
  baz: [object Object] {
    q1: "q1 value"
  },
  foo: [object Object] {
    b1: "overwritten b1",
//--> b2 removed
    b3: "b3 value"
  },
//--> mofo overwritten
  mofo: "overwritten mofo"
}
"merge:"
[object Object] {
  baz: [object Object] {
    q1: "q1 value"
  },
  foo: [object Object] {
    b1: "overwritten b1",
//--> b2 copied
    b2: "b2 value",
    b3: "b3 value"
  },
//--> mofo overwritten
  mofo: "overwritten mofo"
}
"defaults:"
[object Object] {
  baz: [object Object] {
    q1: "q1 value"
  },
  foo: [object Object] {
    b1: "overwritten b1",
//--> b2 copied
    b2: "b2 value",
    b3: "b3 value"
  },
//--> mofo NOT overwritten
  mofo: "mofo value"
}