Hey, I've encountered this need a lot, and lately, I've been toying with the idea of opening a PR about it. But before I do, I want to make sure there really isn't a way of doing what I need using pure Lodash.

Case 1

Functionally add/set a field to an object. We expand on this in case 2.

// Before
_({ 
  name: 'Tomer', 
  age: 23 
})

// After
_({name: 'Tomer'}).extend({age: 23}) // Creates a new object and doesn't modify the original in-place.
// {name: 'Tomer', age: 23}

Case 2

Expanding on case 1, functionally add/set a field based on the object's current values:

// Before
const birthday = moment('10/04/2001');
_({
  name: 'Tomer', 
  birthday, 
  age: birthday.diff(moment(), 'years')
})

// After
_({name: 'Tomer',  birthday: moment('10/04/2001')})
  .extend(obj => ({age: obj.birthday.diff(moment(), 'years')}))
// {name: 'Tomer', birthday: <moment>, age: 23}

Case 3

Expanding on case 2, applying the extend function to each object in an array:

_(people).extend(obj => ({age: obj.birthday.diff(moment(), 'years')}))
// Applies the previously mentioned `extend` to each of the objects in the list
// Basically .map(.extend())

I know there's an existing extend function that modifies the target object in place instead of creating a new object. It also doesn't support dynamic assignment (case 2) or an array equivalent (case 3).

If I would proceed with creating a PR, maybe adding a differently named function will be more fitting.