Difference between map[string]interface{} and interface{}

Question

I want to parse a JSON file to a map[string]interface{}:

var migrations map[string]interface{}
json.Unmarshal(raw, &migrations)

fmt.Println(migrations["create_user"])

But I modified my code to point data to interface{}:

var migrations interface{}
json.Unmarshal(raw, &migrations)

// compile wrong here
fmt.Println(migrations["create_user"])

I don't understand much about difference between map[string]interface{} and interface{} in above case.

Answer 1

The difference between those two types is just what it seems:

1. interface{} is the "any" type, since all types implement the interface with no functions.

2. map[string]interface{} is a map whose keys are strings and values are any type.

When unmarshaling a byte array from JSON into memory it's easiest to use the interface{} type, since it can store any type of JSON document (objects, arrays, primitives, etc.); however, it may require more reflection to handle the underlying data. Using a map[string]interface{} is common when you know the JSON document is an object and []interface{} is common when you know the document is an array.

However, the best approach for unmarshaling JSON - especially when you know the structure of the documents ahead of time - is to define and use custom struct types that describe the data exactly. This way you can avoid any reflection and improve the legibility of your code.