Go's encoding/json stdlib package with some minor enhancements that are very hard achieve without modifying the json package.

This is a drop-in 100% compatible replacement.

Currently included patches:

proposal: encoding/json: "nonil" struct tag to marshal nil slices and maps as non-null (also 37711)

Encode both a nil slice (a slice without a backing array, e.g. var x []string) as [] rather than null, similar to how a slice with a len of 0 is encoded.

This is implemented as a Encoder.NullArray setting for now, which avoids having to add loads of struct tags everywhere. The default is unchanged use NullArray(false) to always encode to JSON []).

proposal: encoding/json: add "readonly" tag

Let's say you have a type like this:

type Foo {
    ID     int    `json:"id"`
    UserID int    `json:"user_id"`
    Name   string `json:"name"`
}

In a REST API, you will probably want to have:

POST /foo      Create a new foo
GET /foo/{id}  Get a foo by ID

The problem is that in the API you want to display all the fields in the GET request, but don't want to allow people to set ID or UserID fields in the POST request.

Turns out this is kind of a tricky problem to solve. You can solve it by creating a new type or a custom json.Unmarshaler, but there are various caveats and it's tricky (see issue for details). With this change, you can add readonly which tells json.Unmarshal to never set these fields:

type Foo {
    ID     int    `json:"id,readonly"`
    UserID int    `json:"user_id,readonly"`
    Name   string `json:"name"`
}