Make setIn write to out-of-bounds array indices

[ditkin]

Running into bug where using setIn to write to out-of-bounds array indices just pushes to array instead.

Code: setIn([2], 'abc', [])
Expected: [empty, empty, 'abc']
Actual: ['abc']

The cause is the use of splice() in _set when dealing with Arrays. Splice doesn’t let you write past the end of an array.

Solution:
instead of using splice we can use next[index] = value, which will fill the array with empty to ensure we write to correct index.

[aem]

One option here to make this change in a backwards-compatible way would be to add this in the current version as a config option, so something like

setIn([2], 'abc', [], { fillMissingKeys: true });

which would result in your [empty, empty, 'abc'] for explicit usages, while keeping the current behavior for existing users. Then if we ever need a new major we could flip that behavior, defaulting that flag to true

[kj800x]

I'm curious, how does this currently work in the case where we have an Immutable.js List? I believe when possible, it makes sense to have a consistent behavior between Lists and native arrays.

[ditkin]

@kj800x Good point - probably why splice was preferred over bracket notation for setting values.