SOURCE

function Foo() {
  this.a = 1;
  this.b = 2;
}
 
Foo.prototype.c = 3;
 
_.forIn(new Foo, function(value, key) {
  console.log('forIn', key);
});

_.forOwn(new Foo, function(value, key) {
  console.log('forOwn', key);
});