[feature request(collections)] option to merge `undefined` in `deepMerge`

[scarf005]

Is your feature request related to a problem? Please describe.

when merging together deeply nested object (e.g react state), it's important to get 'nonempty' values when merging 'empty' values with 'nonempty' values.

for array, map and set it defaults to merging empty and non-empty values together:

deepMerge({ a: new Set([1]) }, { a: new Set() })
//=> { a: Set(1) { 1 } }

however, it doesn't work with nullish values. it'd be very useful to be able to output non-nullish value from merging nullish and non-nullish value together.

deepMerge({ a: 1 }, { a: undefined })
//=> { a: undefined }
// expected: { a: 1 }

Describe the solution you'd like

in DeepMergeOptions, add nullish: MergingStrategy that controls whether deepMerge will choose non-nullish value over nullish ones, such that

type State = {
  pos: {
    x: number
    y: number
  }
}
type UpdateState = Partial<{
  pos: Partial<{ 
    x: number
    y: number 
  }>
}>

const merge = (state: State, newState: UpdateState): State => deepMerge(state, newState, { nullish: "merge" })

always holds. (currently the return type of merge will be UpdateState)

Describe alternatives you've considered

make nullish: "merge” as default. this may cause breaking changes.

[kt3k]

Does nullish: "merge" mean that null also has the same effect as undefined in the above context?

[scarf005]

Does nullish: "merge" mean that null also has the same effect as undefined in the above context?

yes, it could have been nullish: "replace" | "mergeBoth" | "mergeNull" | "mergeUndefined" but the type would be too complex.

[kt3k]

How about only doing this for undefined? (The option would be undefined: "replace" | "ignore"

Handling null in the same way feel a bit strange to me as { a: null } is not assignable to Partial<{ a: number }> for example.

[scarf005]

sure, since Partial only works with undefined.