it mutates *some* of the sources

[sindresorhus]

it mutates *some* of the sources

[schnittstabil]

deep-assign suffers from this behavior for some time – unfortunately, the problem is that, simply recursive Object.assign() does not work as hoped.

The problem is that functions etc. are objects too, and we cannot clone them. Using your example:

const a = {x: () => 42}
const b = {x: {y: 1}}
const result = deepAssign({}, a, b);

console.log(result.x()) // should print 42
console.log(result.x.y) // should print 1

Sorry, I'm not aware of any good solution to this, at the moment a.x is mutated by deep-assign.

If we take a look at the Object.assign Spec, then we see that Object.assign does not distinguish functions etc. from plain objects; therefore there is no (sufficient) reason for deep-assign to do so.

There are many merge tools doing this in a different way, for example:

• _.merge
• merge-options – does not mutate anything (disclaimer: I'm the author of it)

[schnittstabil]

shouldn't it mutate b too?

Well, I do not know. Did you expect that?

[educastellano]

Well no, I wasn't expecting that 😅 Just some coherence, either all or none mutated.

Thanks for the answer @schnittstabil ! :)
I found assign-deep which seems to work well 👍

[schnittstabil]

@educastellano Thanks for the link, I was not aware of that project. The crucial difference of deep-assign and assign-deep can be seen at assign-deep/index.js line 60 using kind-of:

var typeOf = require('kind-of');
// …
function isObject(obj) {
  return typeOf(obj) === 'object' || typeOf(obj) === 'function';
}

The problem I see here is:
The distinction between objects and non-objects is neither based on the behavior of Object.assign nor on what an object in JavaScript is. Therefore it should be is easy to find some unexpected behavior of assign-deep too.

I do not want to go into details of assign-deep, but as far as I can see, it probably would not meet your requirements – assign-deep also mutates sources:

const src1 = {
	b: new Buffer (1),
	p: Promise.resolve(42)
};

const src2 = {
	b: {c: 666},
	p: {q: 666},
};

assignDeep({}, src1, src2);

console.log(src1.b.c) // => 666

// but:
console.log(src1.p.q) // => undefined

[educastellano]

Good to know... It doesn't seem very appropiate to use kind-of for that library. isObject() should say if the value is really an object (not buffer or boolean in case of new Boolean(),...) . /cc @jonschlinkert