Unmarshal JSON with some known, and some unknown field names

Unmarshal twice

One option is to unmarshal twice: once into a value of type Foo and once into a value of type map[string]interface{} and removing the keys "a" and "b":

type Foo struct {
    A int                    `json:"a"`
    B int                    `json:"b"`
    X map[string]interface{} `json:"-"` // Rest of the fields should go here.
}

func main() {
    s := `{"a":1, "b":2, "x":1, "y":1}`
    f := Foo{}
    if err := json.Unmarshal([]byte(s), &f); err != nil {
        panic(err)
    }

    if err := json.Unmarshal([]byte(s), &f.X); err != nil {
        panic(err)
    }
    delete(f.X, "a")
    delete(f.X, "b")

    fmt.Printf("%+v", f)
}

Output (try it on the Go Playground):

{A:1 B:2 X:map[x:1 y:1]}

Unmarshal once and manual handling

Another option is to unmarshal once into an map[string]interface{} and handle the Foo.A and Foo.B fields manually:

type Foo struct {
    A int                    `json:"a"`
    B int                    `json:"b"`
    X map[string]interface{} `json:"-"` // Rest of the fields should go here.
}

func main() {
    s := `{"a":1, "b":2, "x":1, "y":1}`
    f := Foo{}
    if err := json.Unmarshal([]byte(s), &f.X); err != nil {
        panic(err)
    }
    if n, ok := f.X["a"].(float64); ok {
        f.A = int(n)
    }
    if n, ok := f.X["b"].(float64); ok {
        f.B = int(n)
    }
    delete(f.X, "a")
    delete(f.X, "b")

    fmt.Printf("%+v", f)
}

Output is the same (Go Playground):

{A:1 B:2 X:map[x:1 y:1]}

It's not nice, but you could to it by implementing Unmarshaler:

type _Foo Foo

func (f *Foo) UnmarshalJSON(bs []byte) (err error) {
    foo := _Foo{}

    if err = json.Unmarshal(bs, &foo); err == nil {
        *f = Foo(foo)
    }

    m := make(map[string]interface{})

    if err = json.Unmarshal(bs, &m); err == nil {
        delete(m, "a")
        delete(m, "b")
        f.X = m
    }

    return err
}

The type _Foo is necessary to avoid recursion while decoding.

Simplest way is to use an interface like this:

var f interface{}
s := `{"a":1, "b":2, "x":1, "y":1}`

if err := json.Unmarshal([]byte(s), &f); err != nil {
    panic(err)
}

Go Playground example

I use interface to unmarshal uncertain type json.

bytes := []byte(`{"name":"Liam","gender":1, "salary": 1}`)
var p2 interface{}
json.Unmarshal(bytes, &p2)
m := p2.(map[string]interface{})
fmt.Println(m)