property with null value in source object not overwriting to same property in target object

[topgun743]

Hi Dev,
Following is not working correct:

var deepAssign = require("deep-assign");
deepAssign({}, {Lat: null, Lon: null}, {Lat: null, Lon: 2})

output is:
{Lon: 2}

It must be:
{Lat: null, Lon 2}

Due to this weird issue, my form validation is failing as Lat property is totally missing from output.
If I use Object.assign, it works correctly.
Can you plz guide me what is wrong?

[schnittstabil]

Unfortunately, this's on purpose. From MDN:

… null and undefined will be ignored.

More precisely, this is due to the ECMAScript® 2015 Language Specification; 19.1.2.1 Object.assign:

5.a If nextSource is undefined or null, let keys be an empty List.

As deep-assign is Recursive Object.assign(), the following happens:

deepAssign({}, {Lat: null, …}, {Lat: null, …})

// should be similar to
{Lat: deepAssign(<?>, null, null), …}

// which have to be the same as 
{Lat: Object.assign(<?>, null, null), …}

The <?> and the exact recursive semantic is debatable in this case. deep-assign ignores the <?> at the moment – to be at least a bit useful. But in all possible interpretations, the (additional) null values must be ignored because of the spec.

Hence, I believe deep-assign does not match your requirements. There are many merge tools doing this in a different way. Some examples:

• _.merge (2208 loc)
• merge-options (153 loc) – disclaimer: I'm the author of it
• and many more at npm…

[topgun743]

This is strange behavior suggested by ECMA. Null may b a valid and intended value from source. How come not accept it? Also deepAssign must provide an option to even allow such kind of merging.

[schnittstabil]

It would be nice to provide options somehow, but the signature is:

deepAssign(target, source, [source, ...])

If you have an idea, which does not conflict with the semantics of Object.assign, then please let us know.

Btw, we must not provide an option – we are free to do; and we are not ECMA of course 😉. deep-assign is fully dedicated being Recursive Object.assign(); therefore we can also create a new project with a different semantic – like, I did with merge-options.

@topgun743 Anyway, does neither _.merge nor merge-options provide you the semantic you need?

[topgun743]

Looks like I have to turn towards your suggestion. Yes, merge-options looks nice. Will use it now off course. Thanks for suggesting. Great !

[topgun743]

BTW, you can allow a last object as an option config sort of to deepAssign whose first property can be like

{allowNulls: true, . . . . . . other options}

This way it would be assumed that even null values from source should be forced onto the target. I think it should work.

[schnittstabil]

Sadly, because of the Object.assign signature/semantics, these options must be also assigned to the result object, which would be undesirable in my opinion.