Document diffJson() options

[cincodenada]

I ran into this because undefined keys in the diff don't show up, so I needed to supply an undefined replacement

[ExplodingCabbage]

undefined will not be replaced by this function

I don't think this is true. Looking at the code, it seems like the only place that undefinedReplacement gets used is in the default implementation of stringifyReplacer, and that if you provide both, then undefinedReplacement simply gets ignored. There doesn't seem to be any special logic preventing stringifyReplacer from running on an undefined value either:

> diff.diffJson(
...     {x: undefined},
...     {x: "bob"}
... )
[
  { count: 1, added: undefined, removed: true, value: '{}' },
  {
    count: 3,
    added: true,
    removed: undefined,
    value: '{\n  "x": "bob"\n}'
  }
]
> 
> diff.diffJson(
...     {x: undefined},
...     {x: "bob"},
...     {
...         undefinedReplacement: "bob",
...     }
... )
[ { value: '{\n  "x": "bob"\n}', count: 3 } ]
> 
> diff.diffJson(
...     {x: undefined},
...     {x: "bob"},
...     {
...         undefinedReplacement: "jim",
...         stringifyReplacer: (k, v) => k == "x" ? "bob" : v
...     }
... )
[ { value: '{\n  "x": "bob"\n}', count: 3 } ]
> 
> diff.diffJson(
...     {x: undefined},
...     {x: "bob"},
...     {
...         stringifyReplacer: (k, v) => k == "x" ? "bob" : v
...     }
... )
[ { value: '{\n  "x": "bob"\n}', count: 3 } ]

Maybe what you wrote here was true when you opened the PR - dunno, haven't checked - but it's wrong now.