[Feature request] Add filter

[SergeyPirogov]

In order to compare objects, I want to filter some keys. For example:

lhs = {
"name": "a",
"phone": "b"
}

rhs = {
"name": "a",
"phone": "c"
}

diff(lhs, rhs, function filter(node){
   return node.name === 'phone'
})

[anko]

What would the diff call in your example code return? I don't understand yet what "filter" means in this context.

[SergeyPirogov]

@anko objects will be equal because we ignore phone node

[anko]

I see!

I played with some possible workarounds in this Observable notebook trying to see if any of them sound good. Performance-wise, none of them would be as good as having a filter function parameter like this. (Have you tried the alternatives, and found the performance to be a problem?)

Perhaps we should pre-empt all similar future queries by letting the passed-in function instead implement completely custom comparison. Ignoring some property is just a subset of that power:

diff(lhs, rhs, function compare(left, right, propertyName, defaultCompare) {
  // Compare all properties as normal, except `phone`, which always
  // compares as equal.
  if (propertyName === "phone") return true
  else return defaultCompare(left, right, propertyName)
})

That way when someone asks for e.g. case-insensitive string diff, or filtering a property except when null, or things like that, they can implement any such logic.

[papb]

@anko That sounds great!!

[SergeyPirogov]

You can look at some similar library. They have similar filter https://www.npmjs.com/package/deep-diff#changes

[k-funk]

+1

I've come across this deep-object-diff lib in the past few years, hoping to find a lib that does everything this one does, plus filtering. I usually end up writing my own diff, since I find the deep-diff lib that @SergeyPirogov mentioned to be overkill.

I'd prefer the simple key-based filtering api like the one suggested: diff(lhs, rhs, key => key === 'phone'), which is similar to Array.prototype.filter usage. If true, skip comparison+further recursion early. But I'd be amenable to returning defaultCompare if necessary.

Also, while I haven't personally had a use for comparing values, I could see how this "fitler" could also be used for custom comparisons, which might be useful if you wanted to compare Dates, or other complex objects.

---

FWIW, the most common object-diffing usage for me is doing a restful HTTP PATCH that would look something like this:

const PROPS_TO_ALWAYS_INCLUDE = ['id', 'mode'] // could be other identifying keys like 'objectId' or 'mode'

async function patchToServer(urlPath, left, right) {
  const payload = diff(left, right, key => PROPS_TO_ALWAYS_INCLUDE.includes(key))
  await ServerCall(urlPath, 'PATCH', payload)
}

...

patchToServer('/users/user_123', originalData, userModifiedData)

[rdsedmundo]

Also needing this for another use case: ignore word casing.