Easier way to merge into existing object

[NightProgramming]

Sometimes it's useful to not create a new merge result object, but to have an existing object, that is the target of the whole merging process. Right now, I think, I would have to do something like:

// We want to merge newestData into myDataObject:

const result = deepmerge(myDataObject, newestData);
Object.assign(myDataObject, result);

If there was a signature like deepmerge(target, ...sources) the performance might be better because no intermediate object (result) needs to be created. Also there would be less code required (no Object.assign). And if we don't want to merge into any existing object we could do const result = deepmerge({}, myDataObject, newestData));. Note: The return value is the same object we used as first argument. Without having it returned by deepmerge we would need a bit more code: const result = {}; deepmerge(result, myDataObject, newestData);

[NightProgramming]

@RebeccaStevens To me the question arises if the target should also be a source. So when having deepmergeInto(target, source) should the custom mergers receive values from both target and source in order to set the property value of source? I would say yes, because otherwise we would need to call deepmergeInto(target, target, source) if the result value of a target property should be based on its old value + the value of the source property.

So when calling deepmergeInto(target, source) I think the values parameter for custom merger should have two values (one from target property value and one from source. Then for instance we can concat the values and have the result be set as new property value in target.

The only downside I see is that people might expect the values to only come from the sources, but I think that problem can be solved via docs/jsdoc.

[RebeccaStevens]

I agree.
If users don't want values from target, they can use a custom merge version that just ignores the first value in values.