move filter.js to go

[walkingeyerobot]

This would avoid needing to have 2 running processes.

To handle JSON properly in Go, do something like this:

type OptionalInt []*int

func (i *OptionalInt) UnmarshalJSON(data []byte) error {
	if string(data) == "null" {
		*i = []*int{nil}
		return nil
	}

	var x int
	if err := json.Unmarshal(data, &x); err != nil {
		return err
	}
	*i = []*int{&x}
	return nil
}

func (i OptionalInt) MarshalJSON() ([]byte, error) {
	if len(i) == 0 {
		return []byte("null"), nil
	}
	return json.Marshal(i[0])
}

func (i OptionalInt) IsNull() bool {
	return len(i) == 1 && i[0] == nil
}

func (i OptionalInt) IsUndef() bool {
	return len(i) == 0
}

Given the above and some json unmarshalled into this struct:

type Foo struct {
	bar OptionalInt `json:",omitempty"`
}

To set to undefined:

foo.bar = []*int{}

To set to null:

foo.bar = []*int{nil}

To set to any other value:

x := 3
foo.bar = []*int{&x}

In addition to int, this will need to be done for string, bool, and float as well. Keep in mind there are no generics in go, so there will be a lot of repeated code for each type. It'll be ugly. Probably put it in something like jsontypes.go and filter.go, with both extra commented.