Immutable _.assign (assign with clone) from lodash underscore or other library?

Question

Is there an alternative method available in lodash, underscore or other library that almost behaves the same way, except that it returns a new object instead of mutating the first argument?.

var o = { 'user': 'barney' }
var result = method(o, { 'age': 40 }, { 'user': 'fred' })

// o still { 'user': 'barney' }
// result is now { 'user': 'fred', 'age': 40 }

Answer 1

The most common way of doing this seems to use an empty object and assign onto that, like:

var result = _.assign({}, l, m, n, o, p);

This is not technically immutable but will produce a "new" object that did not exist before the function was called.

Bear in mind that even a very clever implementation of clone would have to do this same thing. It's trivial to create the new object manually, so most libraries don't worry about a helper for this case. The next closest thing would be _.create, which has more to do with assigning the correct prototype.

Answer 2

I like defaults() for cases like this.

var user = { user: 'barney', age: 36 };

_.defaults({ age: 40 }, user);
// → { user: 'barney', age: 40 }

user;
// → { user: 'barney', age: 36 }

The first argument is the destination, and user isn't mutated. I like using defaults() when I need to override properties, as is the case here with age, but don't want to actually change anything in the original. Because defaults() will only add properties that resolve to undefined. The age property exists in the object literal, so it's value is kept.

The assign() approach works just as well - defaults() is just a different way to think about it.