Issue regarding null values

[nickydonna]

We have an issue with an special merger. We need to keep the properties even if the source object has that property as undefined, so we use a custom merger, with almost all values they work ok, but when the original object has a value of null, the resulting property is set to undefined.

const Immutable = require("seamless-immutable").static;

const merger = (c, o) => {
    if (o === undefined && c !== undefined) {
      return c;
    }
    return;
}

Immutable.merge({a: 1}, {a: undefined}, {merger}) // {a: 1}
Immutable.merge({a: null}, {a: undefined}, {merger}) // {a: undefined}

Here is the code running on runkit https://runkit.com/donnanicolas/seamless-immutable-null

We don't know why this happens

[crudh]

Seems like a problem in the addToResult function in the merge function: https://github.com/rtfeldman/seamless-immutable/blob/281c344ad4694d4190fbe27d954d5a69887335bf/src/seamless-immutable.js#L405

So if the result from the merger is not truthy it will go to the else and use the value from o. It should do an explicit check for undefined instead. But I guess a fix for that would require a new major version since it can break stuff.

[nickydonna]

@crudh Ok, but it seem like a major issue, since anyone using null values can have issues like this and its rather hard to track

[crudh]

@donnanicolas yeah, I agree. PR #248 should fix it.

[nickydonna]

@crudh Thanks!! will update as soon as the PR is merged!! (I "reviewed" it and it seems great, but I don't know much about the code)

[crudh]

This is fixed now, a release will be done soon.