I had a back and forth with someone who really didn't want to change that behavior and their reasoning was that since you can create and provide an empty map or slice.. having the marshaler do that for you, and then also needing a way to disable that behavior, was unnecessary complexity.

Why shouldn't it be? The nil is null and empty array is an empty array, they are completely different objects.

Not in Go. Empty slices and empty maps are nil, so it's ambiguous.

To be precise.. empty slices and maps sometimes behave like nil (len, range etc) and sometimes not (inserting into a nil map). The former is a neat convenience, and I think extending that to JSON marshaling makes sense.

No, empty slices and empty maps in Go are not nil.

This is the idiomatic way of declaring empty slices in Go, and it prints true:

    var slice []string
    fmt.Println(slice == nil)

Whether to judge the line below idiomatic, or not, is a question I leave to the authorities -- but it is highly convenient, and prints "false".

  slice := []string{}
  fmt.Println(slice == nil)

This is indeed a nil slice, and it does have len() == 0, but Go also has a concept of empty slices separate from nil slices

The language just has a bad habit of confusing them some of the time, but not consistently, so you can still occasionally get bit by the difference

As someone who uses Go a lot, it's just one of those things...

Yes because it's nil. You declared it but not created. Same for map. Same for var something *string

how is a nil map not null? It certainly isn’t a zero-valued map, that would be {}.

The zero value of a map is indeed nil in Go: This prints true (https://go.dev/play/p/8dXgo8y2KTh):

    var m map[string]int
    println(m == nil)

Ok, true!

It should be marshaled into {} by default, with a opt-out for special use cases.

There’s a simple reason: most JavaScript parsers reject null. At least in the slice case.

Not sure what you mean here by "most JavaScript parser rejects null" - did you mean "JSON parsers"? And why would they reject null, which is a valid JSON value?

It's more that when building an API that adheres to a specification, whether formal or informal, if the field is supposed to be a JSON array then it should be a JSON array. Not _sometimes_ a JSON array and _sometimes_ null, but always an array. That way clients consuming the JSON output can write code consuming that array without needing to be overly defensive

Well those are different things, aren't they? Empty slice/map is different from nil. So it makes a lot of sense that nil = null and []string{} = [], and you have an option to use both. That being said, it starts to make less sense if you work with go where the API mostly treats it as equivalent (append, len, []). So that would be my guess how it ended up the way it did.

Also, now that nil map is an empty object, shouldn't that extend to every nil struct that doesn't have a custom marshaller? It would be an object if it wasn't nil after all...

It is different from nil, but then again a nil map in Go behaves like an empty map when reading from it. If you consider serialization to be "reading", therefore, it makes sense to interpret it accordingly.

That is not true, though. Reading from a nil map panics, and reading from an empty map does not.

It doesn't. E.g. this prints 0:

var m map[string]int = nil fmt.Println(m["foo"])

The language spec is also pretty clear on this; https://go.dev/ref/spec#Map_types:

> A nil map is equivalent to an empty map except that no elements may be added.