Proposal: `mergeDeep` about froebel HOT 2 CLOSED

Thanks for the suggestion! I think that would make a great addition to the existing utilities. Will try and find some time this weekend to add it to the library.

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It's implemented under the name merge by bd90ce2 and included in v0.23.0.

That was way more interesting than expected because of all the special cases that need to be considered.

For example, merging self-referential data:

const a = {}
const b = {}
a.foo = b
b.bar = a
const merged = merge(a, b)

What should merged be? Clearly { foo: merged, bar: merged }.

But what about this?

const a = [1, 2]
a.push(a)
const b = [3, 4]
b.push(b)
const merged = merge(a, b)

Probably [ 1, 2, merged, 3, 4, merged ] (that's how it's implemented) though I'd understand if someone argues merged[2] and merged[5] shouldn't be the same reference.

And what about references in Maps?

const a = new Map()
a.set([a, a])
const b = new Map()
b.set([b, b])
const merged = merge(a, b)

I don't think there is any right or wrong answer here but I decided self-references in both keys and values will also be self-referencing in the result, so [...merged.entries()] will be [ [merged, merged] ].
For some reason in the last example Lodash will return [ [a, a], [merged, merged] ] which I don't think makes any sense at all.

There were also some interesting problems in the type signature. E.g. what is the type of a when merging { a: 1 } and { a?: 2 }? It could be 1 or 2 or undefined (if the second object is { a: undefined }) but it shouldn't be optional because there will always be a value for a in the first object, so the resulting type should be { a: 1 | 2 | undefined }.

And what about if one or both of the merged values is a union type? E.g. merging string | { a: 1; d: 4 } and number | { a: 2; c: 3 }. All the possible types of the unions must be considered so the resulting type has to be number | { a: 2; c: 3 } | { a: 2; c: 3; d: 4 }.

And what about something like merge({ list: [1, 2] }, { list: new Set([2, 3]) })? Should arrays and Sets be merged? Same could be asked for regular objects and Maps or other iterable structures. For now I decided to treat them as separate types so if the first parameter is an array and the second one a Set then the Set will override the array and their values won't be merged. But I think it might make sense to add the option to define custom merge strategies for these cases in the future.

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