Parsing arbitrary JSON documents

Parsing into a struct is very convenient but sometimes you don’t know the structure of JSON document upfront.

For arbitrary JSON documents we can decode into a map[string]interface{}, which can represent all valid JSON documents.

var jsonStr = `{
	"name": "Jane",
	"age": 24,
	"city": "ny"
}`

var doc map[string]interface{}
err := json.Unmarshal([]byte(jsonStr), &doc)
if err != nil {
	log.Fatalf("json.Unmarshal failed with '%s'\n", err)
}
fmt.Printf("doc: %#v\n", doc)
name, ok := doc["name"].(string)
if !ok {
	log.Fatalf("doc has no key 'name' or its value is not string\n")
}
fmt.Printf("name: %#v\n", name)

doc: map[string]interface {}{"age":24, "city":"ny", "name":"Jane"}
name: "Jane"

For basic JSON types, the value in the map is bool, int, float64 or string.

For JSON arrays, the value is []interface{}.

For JSON dictionaries, the value is (again) map[string]interface{}.

This approach is flexible but dealing with map[string]interface{} to access values is rather painful.