merge can mutate source

[Xiot]

I found some unexpected behaviour with the merge function.

It appears that when values are merged, there are cases where it just copies the reference to the source object.

const target = {}
const source = { a: { b: 1 } }
const m = merge(target, source)
console.log('merged', m)
console.log('target', target)
console.log('source', source)

m.a.c = 2
console.log('merged', m)
console.log('target', target)
console.log('source', source)

which produces

merged { a: { b: 1 } }
target { a: { b: 1 } }
source { a: { b: 1 } }
merged { a: { b: 1, c: 2 } }
target { a: { b: 1, c: 2 } }
source { a: { b: 1, c: 2 } }  // expected `source` to not have changed

This seems to be what is responsible https://github.com/toss/es-toolkit/blob/main/src/object/merge.ts#L98

The same code using the merge method from lodash produces

merged { a: { b: 1 } }
target { a: { b: 1 } }
source { a: { b: 1 } }
merged { a: { b: 1, c: 2 } }
target { a: { b: 1, c: 2 } }
source { a: { b: 1 } }

At least in my eyes, the one from lodash seems more correct, since the source is never mutated.
Curious to hear your thoughts on if this is intentional, and if so, what is the reasoning.

[Xiot]

Just noticed that there is a PR that addresses this https://github.com/toss/es-toolkit/pull/556

[raon0211]

Hello, thanks for your report. We merged the pull request. https://github.com/toss/es-toolkit/blob/main/src/object/merge.ts#L98

Can you check if es-toolkit@1.22.0-dev.706 works for you?

[Xiot]

es-toolkit@1.22.0-dev.706 seems to fix the issue.

Thanks