Model constructor modifies descriptor object about mobx-state-tree

[dangreen]

why not just copy object before reduce it?

return keysList.reduce((props, key) => { ... }, { ...declaredProps })

Before optimization object was copied on every loop step, in that way it will be copied only once.

[dangreen]

@coolsoftwaretyler @jamonholmgren fyi

[coolsoftwaretyler]

Ah good point, @dangreen. I don't think we considered the use case where you'd want to have the original object stay un-modified. Most of the time, I think of model instantiation like:

const Model1 = types.model('Model1', { num: 0, str: ''})

In which case, the in-place modification wouldn't be a problem. I can see how you would consider this a breaking change, though we certainly didn't. It can be tough to determine the right classification for changes in MST, where there is a lot of undocumented and untested behavior.

I really prefer the changed version of this, but I wonder if we can put it behind an opt-in flag instead, to restore the prior behavior, but allow people to get the benefits of faster model instantiation when they know their second parameter is safe to modify.

I'll label this as a bug and we'll see what we can do here. Thanks for the issue, and would be happy to review a PR!

[coolsoftwaretyler]

That works, too! I was just writing my first idea, haha. I like yours better.

Do you want to open a PR to that effect?

[coolsoftwaretyler]

This fix is available in 5.4.2 now: https://github.com/mobxjs/mobx-state-tree/releases/tag/v5.4.2, will be shipped with v6 as well.