Accessing JSON properties when not using struct

I have some JSON and I need to be able to access the properties. As the JSON properties can vary I can't create a struct to unmarshal into.

Example

The JSON could be this:

{"name" : "John Doe", "email" : "john@doe.com"}

or this:

{"town" : "Somewhere", "email" : "john@doe.com"}

or anything else.

How can I access each of the properties?

You can unmarshal it into an interface{}. If you do that, json.Unmarshal will unmarshal a JSON object into a Go map.

For example:

var untypedResult interface{}
err := json.Unmarshal(..., &untypedResult)

result := untypedResult.(map[string]interface{})

// ... now you can iterate over the keys and values of result ...

See <http://blog.golang.org/json-and-go#TOC_5.> for a complete example.

If you just happen to have fields that may not be specified, you can unmarshal your input into a struct with pointer. If the field isn't present, the pointer will be nil.

package main

import (
	"encoding/json"
	"fmt"
)

type Foo struct {
	A *string
	B *string
	C *int
}

func main() {
	var input string = `{"A": "a","C": 3}`
	var foo Foo
	json.Unmarshal([]byte(input), &foo)
	fmt.Printf("%#v\n", foo)
}

Playground

If you really want something more flexible, you can also unmarshal your input into a map[string]interface{}.

package main

import (
	"encoding/json"
	"fmt"
)

func main() {
	var input string = `{"A": "a","C": 3}`
	var foo map[string]interface{} = make(map[string]interface{})
	json.Unmarshal([]byte(input), &foo)
	fmt.Printf("%#v\n", foo)
}

Playground

When unmarshalling a JSON document, not all properties that are defined in the struct need to be present in the JSON document. In your example, you could define the following struct:

type MyJson struct {
    Name string `json:"name"`
    Town string `json:"town"`
    Email string `json:"email"`
}

When your use this struct to unmarshal a JSON document that has one or more of these properties missing, they will be initialized with the respective type's null value (an empty string for string properties).

<hr>

Alternatively, you can use the generic interface{} type for unmarshalling and then use type assertions. This is documented in-depth in the blog post "JSON and GO":

var jsonDocument interface{}

err := json.Unmarshal(jsonString, &jsonDocument)
map := jsonDocument.(map[string]interface{})
town := map["town"].(string);