Target object values of `undefined` result in invalid patch

[sean256]

runkit: https://runkit.com/embed/t341yubb8g39

var { createPatch, applyPatch } = require("rfc6902")

const a = { name: 'bob', image: undefined, cat: null }
const b = {name: 'bob', image: 'foo.jpg', cat: 'nikko' }

const patch = createPatch(a,b);

console.log(patch);

After applying the patch to a image is NOT updated to foo.jpg. If I exclude image instead of having it as undefined it works as it creates an add operation instead of replace. This is causing me some headaches right now as a framework I'm stuck with generates undefined values. When passing these objects over JSON the undefined values are stripped which really makes the replace nonsense.

If anyone else is having this issue I found a workaround by purging all undefined properties

const cleanUndefined = object => JSON.parse(JSON.stringify(object));

[chbrown]

Good catch! I wish I'd read your issue more closely the first go-round :( ... the solution I eventually arrived at (after a hack that didn't really get at the core issue), follows the same logic :)

The problem was that op=replace is like op=remove followed by op=add, but op=remove is supposed to fail if the target location doesn't "exist". It's not exactly clear when a "target location" exists or not; is it when there's a value defined there? or simply when the path to get there exists?

Suppose we have a JSON Pointer, p, with path "/k", in the context of an object obj. Then, the target location indicated by p "exists" iff:

1. obj.hasOwnProperty('k') returns true (in which case the existence check in patch.remove and patch.replace should fail on !parent.hasOwnProperty(key) instead of value === undefined)

or, alternatively:

2. obj.hasOwnProperty('k') returns true and obj.k is not strictly equal to undefined (which is based on how JSON treats undefined — as you pointed out with your workaround that cleans the value with a JSON.parse ∘ JSON.stringify cycle)

The solution in a87214a implements the latter (2). To reuse your sample, the new behavior looks like this:

const {createPatch, applyPatch} = require('rfc6902')
const a = {name: 'bob', image: undefined, cat: null}
const b = {name: 'bob', image: 'foo.jpg', cat: 'nikko'}
createPatch(a, b)
//=> [ { op: 'add', path: '/image', value: 'foo.jpg' },
//=>   { op: 'replace', path: '/cat', value: 'nikko' } ]
applyPatch(a, createPatch(a, b))
//=> [ null, null ]
a
//=> { name: 'bob', image: 'foo.jpg', cat: 'nikko' }

I haven't published a new version yet, but I think a behavior change this deep (albeit subtle, and clearly an improvement in correctness) merits incrementing the major version. Lemme think on that.

[sean256]

Nice! I'm surprised no-one came across this yet. I would agree that this is probably a major version despite how simple it is :-/ I could totally see this "fixing" something that someone wasn't even aware was broken and it causing trouble.

By the way, I want to call out how awesome this module is. I tried almost every patch library out there and yours consistently generates the smallest patches!