You can use the delete operator, although I don't particular recommend using that alternative. If you do use it, you need to be aware of it's limitations and how it behaves if you're using strict mode.
If you instead want an immutable alternative, you can use, like you say, lodash. What you'd do then is use _.omit (for creating a new object with the given properties omitted) or _.pick (for creating a new object with only the given properties included). Like this:
var obj = {
firstname: "Nikolaj",
lastname :"Larsen",
age:99
};
var result = _.omit(obj , ['age']);
// result: { firstname: .., lastname: .. }
and
var obj = {
firstname: "Nikolaj",
lastname :"Larsen",
age:99
};
var result = _.pick(obj , ['firstname', 'lastname']);
// result: { firstname: .., lastname: .. }
Like said, it's immutable so it doesn't change the old object.