When a particular property of the source object is undefined while the target one has a value, the value will be override by undefined

[PhoenixMenia]

the below code will get a return value showed in the picture below. I dont want an invalid value to override the value of the target.

var deepmerge = require("deepmerge")

const combineMerge = (target, source, options) => {
if (typeof source[0] === 'string') {
return source;
}

const destination = target.slice()

source.forEach((item, index) => {
    if (typeof destination[index] === 'undefined') {
        destination[index] = options.cloneUnlessOtherwiseSpecified(item, options)
    } else if (options.isMergeableObject(item)) {
        destination[index] = deepmerge(target[index], item, options)
    } else if (target.indexOf(item) === -1) {
        destination.push(item)
    }
})
return destination

}

const options = {
arrayMerge: combineMerge,
customMerge: (key, options) => {
return undefined
}
}

const alex = {
name: {
first: 'Alex',
last: 'Alexson'
},
pets: ['Cat', 'Parrot']
}

const tony = {
name: {
first: undefined,
last: 'Tonison'
},
pets: ['Dog']
}

const result = deepmerge(alex, tony, options)

[RebeccaStevens]

I believe this is by design. You'd need to use define a custommerge function if you want to change this behavior.

This is also by design in my library (deepmerge-ts) but this behavior can be overridden. Here's the example of doing that: RebeccaStevens/deepmerge-ts#25

[PhoenixMenia]

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