Check if JSON is Object or Array

Question

Is there a simple way in Go to check whether given JSON is either an Object {} or array []?

The first thing that comes to mind is to json.Unmarshal() into an interface, and then see if it becomes a map, or a slice of maps. But that seems quite inefficient.

Could I just check if the first byte is a { or a [? Or is there a better way of doing this that already exists.

Answer 1

Use the following to detect if JSON text in the []byte value data is an array or object:

 // Get slice of data with optional leading whitespace removed.
 // See RFC 7159, Section 2 for the definition of JSON whitespace.
 x := bytes.TrimLeft(data, " \t\r\n")

 isArray := len(x) > 0 && x[0] == '['
 isObject := len(x) > 0 && x[0] == '{'

This snippet of code handles optional leading whitespace and is more efficient than unmarshalling the entire value.

Because the top-level value in JSON can also be a number, string, boolean or nil, it's possible that isArray and isObject both evaluate to false. The values isArray and isObject can also evaluate to false when the JSON is invalid.

Answer 2

Use a type switch to determine the type. This is similar to Xay's answer, but simpler:

var v interface{}
if err := json.Unmarshal(data, &v); err != nil {
    // handle error
}
switch v := v.(type) {
case []interface{}:
    // it's an array
case map[string]interface{}:
    // it's an object
default:
    // it's something else
}