mergeIn throws Cannot read property 'merge' of null when mergeIn on keyPath that has a null value

[Vetrano89]

What happened

mergeIn throws Cannot read property 'merge' of null when mergeIn on keyPath that has a null value

An error is also thrown when a field is undefined, which makes more sense. To me, null is a placeholder and says "yes there will eventually be a value here" and that value can be anything. So why can't we merge into it?

It looks like mergeIn works on other values that aren't objects such as Bool/Nums etc. so it doesn't seem like this is intentional.

How to reproduce

https://codepen.io/vetrano89/pen/xxRGaJE

[jdeniau]

The error message is better in the latest 4.0.0-rc:

TypeError: Cannot merge into non-data-structure value: null

The mergeIn will try to merge at the path you specify, so you will try to merge null and Map({foo: 'bar'}), which is not possible because null can not be merged.

This is arguable thought, because it will work with an undefined value :

map1.mergeIn(['foo'], map2);
// will output
{
  foo: {foo: "bar"}
  nullField: null
}

[jdeniau]

Probably related to the discussion in #1840 for resolution

[Methuselah96]

Took a brief look at this and I think it will require its own PR/fix separate from #1840.