Why would deepmerge assign 'undefined' attributes?

[ivawzh]

Currently deepmerge overwrites non-void attribute with undefined attribute:

$ node
const merge = require('./node_modules/deepmerge')
merge({ a: 'a' }, { a: undefined })
# Result: { a: undefined }

Why is this a desired behaviour?

IMHO, the conventional merge method should skip undefined value. The popular libarary Lodash also aggrees that rule, see http://stackoverflow.com/a/33247597/1401634.

Would you please at least provide an option to turn this behaviour off?

Cheers

[TehShrike]

deepmerge operates on all properties of the input objects, no matter what their value is.

It may be that most people would assume the opposite - though I see that lodash's assign and extend behave the same way.

Making a breaking change for something fairly opinionated like this seems untenable.

I worry about feature creep, but I could see the argument for this.

I wouldn't be against a pull request adding a new mergeUndefined option.

We could also talk about a general solution - allowing users to pass in a shouldOverwriteSource function that would take the values from both objects, and determine if the merging object should be populated from the source object or the target object. It would default to () => true for backwards compatibility.

Anyone have thoughts?

[macdja38]

I think if you can gather anything from the lodash example it's that both functionalities can be valuable. undefined is a possibly desirable value just like any other. I support addition of a custom function the user can define to dictate merge functionality, that said assuming the need for undefined is a common case I would also like to see a function that supports it out of the box.

[yvele]

Is there a workaround or and options not to merge undefined values?

[TehShrike]

Yeah, you can pass in a customMerge function: https://github.com/TehShrike/deepmerge#custommerge

Something like

const customMerge = key => (a, b) => b === undefined ? a : deepmerge(a, b)

[yvele]

@TehShrike unfortunately your suggestion is not working 🤔
But once a working solution has been found, I think I could be interesting to add it in the readme

[yuccai]

This should work :

const customMerge = () => (a, b) => {
  const c = Object.keys(b)
    .filter([key, value] => value !== undefined) 
    .reduce((obj, [key, value]) => ({
      ...obj,
     key : value,
    }, {});
  return deepmerge(a, c);
};

[pSnehanshu]

customMerge never being called #199

[tran-simon]

@TehShrike Could this issue be reopened? We're having the same issue and the suggested solutions do not work